Why must I always close all other programs before installing a new program?

Not as important anymore, but not a horrible idea.

Basically all installed programs use some helpers from the operating system to get things done. If some of those helpers can’t be accessed because they’re in use during an installation then things might get mucked up.

Most programs now have a check and then can wait until the file is no longer in use (or force the other program to release it for a moment) but it wasn’t always that way and I’m sure there’s still situations where it would cause problems.

At this point, it’s mostly insurance to quit everything, but really cheap and easy insurance, so why not?

As @funkdaddy stated.

In simple terms:
It’s like two ladies trying to use the same toilet at the same time – It can be done but prepare for possible issues.

I was typing out a longer, more detailed, answer ..but realized it was very similar to @funkdaddy‘s response.

It it’s a suggestion from the program your installing then it’s either to insure the installation process or to save from having to reboot once it’s installed.. If you have dial-up it’s particularly important as you need all the resources your system can manage in order for a successful installation. If you don’t mind rebooting then you don’t need to close everything even if it advises it.

I stopped doing this years ago, and have never once had a problem.

As noted above potentially shared resources being updated.

In many cases at a fundamental level in the Operating System, currently running programs are using files that must be updated. So, for example, when you go to install a Java update, you will be absolutely required to close any program that’s already running in Java, because it’s using files that will probably be updated with new versions. Since Windows won’t allow a file to be saved on top of an already open file, the update can’t proceed until the Java file is closed.

I generally don’t bother unless the installation fails the first time I try if because certain files were in use.