What does 'git blame' do?

I saw a lot of questions about methods of using git blame, but I don't really understand them.

I see a Blame button on top of files on the GitHub interface. Upon clicking it, it shows some diff with usernames on the left bar. What does that indicate?

Why is git blame actually used, apart from GitHub?

From git-blame:

Annotates each line in the given file with information from the revision which last modified the line. Optionally, start annotating from the given revision.

When specified one or more times, -L restricts annotation to the requested lines.

Example:

[email protected]:~# git blame .htaccess
...
^e1fb2d7 (John Doe 2015-07-03 06:30:25 -0300  4) allow from all
^72fgsdl (Arthur King 2015-07-03 06:34:12 -0300  5)
^e1fb2d7 (John Doe 2015-07-03 06:30:25 -0300  6) <IfModule mod_rewrite.c>
^72fgsdl (Arthur King 2015-07-03 06:34:12 -0300  7)     RewriteEngine On
...

Please note that git blame does not show the per-line modifications history in the chronological sense. It only shows who was the last person to have changed a line in a document up to the last commit in HEAD.

That is to say that in order to see the full history/log of a document line, you would need to run a git blame path/to/file for each commit in your git log.

It's to figure out which co-worker wrote the specific line or ruined the project, so you can blame them :)

From GitHub:

The blame command is a Git feature, designed to help you determine who made changes to a file.

Despite its negative-sounding name, git blame is actually pretty innocuous; its primary function is to point out who changed which lines in a file, and why. It can be a useful tool to identify changes in your code.

Basically, git-blame is used to show what revision and author last modified each line of a file. It's like checking the history of the development of a file.

The git blame command is used to know who/which commit is responsible for the latest changes made to a file. The author/commit of each line can also been seen.

git blame filename (commits responsible for changes for all lines in code)

git blame filename -L 0,10 (commits responsible for changes from line "0" to line "10")

There are many other options for blame, but generally these could help.