I'll just be playing to the chorus here, but I'm also in the process of creating my own tribe and just wanted to give my own perspective. In addition to all of the reasons and resources people have already given above, I personally just really enjoy world building and character creation! What I get out of the process is the sheer enjoyment of researching various cultures to pick and choose influences from, to blend them together in a neat way. Even if I don't actually use most of what I read/watch about, simply learning about things I had no idea of previously is part of the adventure.
It can lead to tons of potentially interesting RP, so definitely don't shy away from making something personalized and unique.
It can lead to tons of potentially interesting RP, so definitely don't shy away from making something personalized and unique.