system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you install a piece of software, why are you told to restart your computer even though it runs fine without restarting it?
gpt
This is a big oversimplification, but here we go.

If a file is in use by the filesystem, what the software has control of is a file handle: it's an address to where the file is located on disc. When you update something or overwrite a file, the operating system creates a new file, and references the filename to the new file's file handle.

The old file, however, is still marked as in use. It's still there on disc, and _that_'s the file being accessed by currently running software. To fix this, software updaters or installers will often have you _close only the software_ that's being used. Think about updating any random piece of software: you have to close it to update it more often than not, right?

Recall that, since the program itself is a file (e.g. an .exe file), when you update software, you expect to have the new version of the software running, and not the old one. The only way to ensure that all the new files are being used is to close that software and reopen it. When you do this, you get the new version of the software running.

Now, extend that idea to files that the _operating system_ needs. The only way to ensure the operating system is truly up to date is to close it and reopen it. This is a computer restart.

So, tl;dr: it's not _actually_ updated until you restart, you just have the new files.