I think others have answered what the folder should do.
FSearch is great, but I wouldn’t index the entire file system. There isn’t much point in indexing things you won’t be using such as all the system files and the representations of hardware processes. It’s a bit like on Windows indexing c:\windows - you just don’t need all that clogging up your search results. But the Linux filesystem encompasses much more so you’d get even more stuff.
On my system I index my home folder (where all your own files will be kept) and my mount points (for me a series of drives I mount under /mnt/). You could also index /media (or variants) as that is where USB drives, and CDs etc would mount to - but I don’t tend to index USB sticks etc.
I can see circumstances where you might want to index other locations depending on how you use fsearch and Linux, but I think for most users it’d just be unnecessary indexing and results.
Edit: I saw someone else mention /etc too. That can be useful if you want to find system config files. They also mentioned /usr/share/docs which contains a lot of the Linux manual/distro docs amongst others. If you want to access that then it’s not a bad idea to index it, although most people are online all the time now on multiple devices so it may be a bit redundant for most users day to day; I tend to just search online documentation.