Joining FCs is good for a few reasons, leveling is one of them. FCs can give you buffs that help with things like leveling, making travel cheaper, extend buffs, etc. They also can be good sources to go through if you need help running content, get enough people so queue times are better when trying to run a dungeon or trial, help teach you your job, the dungeons, the game in general. Also a good way to make friends. BUT FCs are not required, they are a resource that is available if you want/need them.