I think they are necessary to scale the platform by creating these communities with their own reward system we allow for easier digestible pieces of the blockchain where they can feel more in control, welcome users and share in certain thoughts an opinions
I believe steem needs tribes if it is to become the platform for everyone. We are now in a position to create multiple niche audiences where users can hang out and share their content and thoughts and hope over to others they like too]
It's a unique shared universe and I feel could be that really eventually takes this place into the eyes and devices of more people