Leveling is how your association grows!
When you do your library sorts and commissions each day, you get points towards growth. You also get even more points for donating books with diamonds.
The benefits of leveling are:
More people doing commissions will help you reach those 5 star commission rewards
Unlock co-op suits at Level 4
The higher your guild level, the more materials you gain for co-op!
More activity means it's easier to rank, if that's one of your goals