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?