Some do use this option! I use it to keep track of those I fish with or talk to a lot or people I consider my "friends". This function allows me to find them quick to shoot off a PM, see when they have been on if I'm looking for them for one reason or another.
If you are not on my list doesn't mean your not my friend its just that I probably don't interact with you a lot. I find it a useful function for keeping track of my friends!