My wife's friends/family use them as a form of silo'ing different groups of people. I haven't used FB in years but there used to not really be a good way to do that, at least. That would be my guess. For example, my wife is friends with her own sister 3 times as 3 slightly varying names.

I won't get into my mom's use case. I think she mainly uses them to friend with and snoop on people. Weird lady.

I use one for my personal use. Another for video calls for my kid. Another for snooping on girls I slept with when I was single. A fourth for apps that want third party login.