Guilds hold the secrets of the strongest magic. They have developed specialization. They have the institutional knowledge. They hold lock and key to power. You learn spells from the guild, and best practices.
All this power and knowledge is secret. Anyone within the guild is beholden to the power and knowledge. You need to keep the secrets and follow the rules. If you don’t, you are forfeit. You will be rounded up and brought to trial. If you fight the summons to court, your life is forfeit. Trial is at the Ensource.
The guilds were designed to be beholden to the Ensource governing structure, but this is no longer the case. At one point, there was a revolution within magi society. This occurred at the same moment when other peoples besides the Angen were allowed to become initiates and, thus, magi.
The Ensource leadership is chosen by leaders of the guilds.