Does it actually matter whether you have open applications when installing new software?

It seems the norm these days is for installers/setup programs to request that you close all open applications before initiating the install process for a piece of new software. I used to obediently follow these directions without fail, even though it could sometimes be frustrating having to close open documents and stop working on things just to get a new, seemingly unrelated application installed. Then at some point I simply stopped bothering. Nowadays if I have a lot of stuff going on I might even run multiple installers at the same time; I can't even recall a time it has ever posed a problem.

Why do setup programs even make this request in the first place, then, when it appears to be unnecessary? Is this just to simplify troubleshooting for companies' support people? Has anyone else ever run into problems as a result of trying to install an app while other apps were open?

Sometimes it does, sometimes it doesn't matter.

The installer may require access to registry keys that could be locked by an open program.

Also, some installers create a system restore point, open programs can cause problems.

For example, try installing Adobe Flash Player with your web browser open.

Good computing practice is to close running applications before installing any software.

Most times it doesn't, but sometimes it does. For example, old versions of Rhinoceros3d wouldn't install if you had Windows Explorer open (I think they solved that issue now, but I'm not sure).

It is a rather rare issue nowadays, but the warning still remains.

Heh, an old one from bash.org

#139697 +(8107)- [X]

<frank> can you help me install GTA3?
<knightmare> first, shut down all programs you aren't using
frank has quit IRC. (Quit)
<knightmare> ...

Well-written installers will prompt you to close specific applications that are getting in the way. Even some poorly-written installers will give you enough clues to figure out what is in conflict. Just closing applications that you suspect to conflict (web browser while installing a browser plugin) is sufficient most of the time.

Installers often update system files which may be in use causing an installation to either fail, or require a reboot to complete fully.

By closing all applications, the installer can overwrite system files without any trouble and complete successfully so the newly installed program can be launched either straight away, or after a reboot (depending on the nature of the update).

Installers can't (or more previsely they could but shouldn't and won't do usually) replace DLLs and other files that are in use by another process. That's why most installers suggest that you should close all open applications. The only consequence when you leave them open is that you'll have to restart after the installation—because the problematic files will then be replaced on the next boot.

Many programs however, don't replace that many DLLs used by other processes and as such it doesn't matter most of the time.

But as others noted, there are some processes that must be closed for the installer to continue, depending on what you're installing.