A fellowship activates tournaments and fellowship adventures. Tourneys are the best source of relics, runes, and KPs. Fellowship Adventures are a good source of finishing out artifacts of recent evolve buildings from events. Spire, although accessible if you play by yourself, is most rewarding when done with a fellowship and best source of free diamonds. The most important aspect of a fellowship is probably extending your trading network by up to another 24 players without fees. It’s pretty critical early game for regular goods, but sentient goods on are traded server wide so is less critical.
When in a well fitting fellowship, you get to play with like minded players all working towards the same goal. You make new friends and learn about random cultures in between some really slow collections of late chapters. There is a lot of camaraderie and teamwork. When in a poor fitting fellowship, you feel like you are with people playing a different game. If you are a casual player in a competitive fellowship, it feels really stressful meeting expectations. If you are a competitive player in a casual fellowship, it feels like you’re hanging out with a bunch of slackers, moochers, and/or deadwood. If you are an Archmage running a fellowship, it can be really rewarding rallying 25 very different people to somehow work together to get the same goal accomplished OR it can feel like total drama babysitting man-babies.
In short: your mileage may vary.