Why does git perform fast-forward merges by default?

Fast-forward merging makes sense for short-lived branches, but in a more complex history, non-fast-forward merging may make the history easier to understand, and make it easier to revert a group of commits.

Warning: Non-fast-forwarding has potential side effects as well. Please review https://sandofsky.com/blog/git-workflow.html, avoid the 'no-ff' with its "checkpoint commits" that break bisect or blame, and carefully consider whether it should be your default approach for master.

^{(From nvie.com, Vincent Driessen, post "A successful Git branching model")}

Incorporating a finished feature on develop

Finished features may be merged into the develop branch to add them to the upcoming release:

$ git checkout develop
Switched to branch 'develop'
$ git merge --no-ff myfeature
Updating ea1b82a..05e9557
(Summary of changes)
$ git branch -d myfeature
Deleted branch myfeature (was 05e9557).
$ git push origin develop

The --no-ff flag causes the merge to always create a new commit object, even if the merge could be performed with a fast-forward. This avoids losing information about the historical existence of a feature branch and groups together all commits that together added the feature.

Jakub Narębski also mentions the config merge.ff:

By default, Git does not create an extra merge commit when merging a commit that is a descendant of the current commit. Instead, the tip of the current branch is fast-forwarded.
When set to false, this variable tells Git to create an extra merge commit in such a case (equivalent to giving the --no-ff option from the command line).
When set to 'only', only such fast-forward merges are allowed (equivalent to giving the --ff-only option from the command line).

The fast-forward is the default because:

• short-lived branches are very easy to create and use in Git
• short-lived branches often isolate many commits that can be reorganized freely within that branch
• those commits are actually part of the main branch: once reorganized, the main branch is fast-forwarded to include them.

But if you anticipate an iterative workflow on one topic/feature branch (i.e., I merge, then I go back to this feature branch and add some more commits), then it is useful to include only the merge in the main branch, rather than all the intermediate commits of the feature branch.

In this case, you can end up setting this kind of config file:

[branch "master"]
# This is the list of cmdline options that should be added to git-merge 
# when I merge commits into the master branch.

# The option --no-commit instructs git not to commit the merge
# by default. This allows me to do some final adjustment to the commit log
# message before it gets commited. I often use this to add extra info to
# the merge message or rewrite my local branch names in the commit message
# to branch names that are more understandable to the casual reader of the git log.

# Option --no-ff instructs git to always record a merge commit, even if
# the branch being merged into can be fast-forwarded. This is often the
# case when you create a short-lived topic branch which tracks master, do
# some changes on the topic branch and then merge the changes into the
# master which remained unchanged while you were doing your work on the
# topic branch. In this case the master branch can be fast-forwarded (that
# is the tip of the master branch can be updated to point to the tip of
# the topic branch) and this is what git does by default. With --no-ff
# option set, git creates a real merge commit which records the fact that
# another branch was merged. I find this easier to understand and read in
# the log.

mergeoptions = --no-commit --no-ff

The OP adds in the comments:

I see some sense in fast-forward for [short-lived] branches, but making it the default action means that git assumes you... often have [short-lived] branches. Reasonable?

Jefromi answers:

I think the lifetime of branches varies greatly from user to user. Among experienced users, though, there's probably a tendency to have far more short-lived branches.

To me, a short-lived branch is one that I create in order to make a certain operation easier (rebasing, likely, or quick patching and testing), and then immediately delete once I'm done.
That means it likely should be absorbed into the topic branch it forked from, and the topic branch will be merged as one branch. No one needs to know what I did internally in order to create the series of commits implementing that given feature.

More generally, I add:

it really depends on your development workflow:

• if it is linear, one branch makes sense.
• If you need to isolate features and work on them for a long period of time and repeatedly merge them, several branches make sense.

See "When should you branch?"

Actually, when you consider the Mercurial branch model, it is at its core one branch per repository (even though you can create anonymous heads, bookmarks and even named branches)
See "Git and Mercurial - Compare and Contrast".

Mercurial, by default, uses anonymous lightweight codelines, which in its terminology are called "heads".
Git uses lightweight named branches, with injective mapping to map names of branches in remote repository to names of remote-tracking branches.
Git "forces" you to name branches (well, with the exception of a single unnamed branch, which is a situation called a "detached HEAD"), but I think this works better with branch-heavy workflows such as topic branch workflow, meaning multiple branches in a single repository paradigm.

Let me expand a bit on a VonC's very comprehensive answer:

First, if I remember it correctly, the fact that Git by default doesn't create merge commits in the fast-forward case has come from considering single-branch "equal repositories", where mutual pull is used to sync those two repositories (a workflow you can find as first example in most user's documentation, including "The Git User's Manual" and "Version Control by Example"). In this case you don't use pull to merge fully realized branch, you use it to keep up with other work. You don't want to have ephemeral and unimportant fact when you happen to do a sync saved and stored in repository, saved for the future.

Note that usefulness of feature branches and of having multiple branches in single repository came only later, with more widespread usage of VCS with good merging support, and with trying various merge-based workflows. That is why for example Mercurial originally supported only one branch per repository (plus anonymous tips for tracking remote branches), as seen in older revisions of "Mercurial: The Definitive Guide".

Second, when following best practices of using feature branches, namely that feature branches should all start from stable version (usually from last release), to be able to cherry-pick and select which features to include by selecting which feature branches to merge, you are usually not in fast-forward situation... which makes this issue moot. You need to worry about creating a true merge and not fast-forward when merging a very first branch (assuming that you don't put single-commit changes directly on 'master'); all other later merges are of course in non fast-forward situation.

HTH