I view wizard guilds like medieval guilds. They control the standards of training and set minimum rates for magic items, spell casting ect., and control the sale of such items. You don't have to be a member of a guild if you are just passing through, but if you use magic then you are subject to a whole slew of laws that don't appply to guild members. These laws are enforced by the guild, not some level 1 watchman. The guilds have a strong interest in protecting their monopoly.
If you become resident in an area controlled by a guild, you still don't need to join but often must register. Failure to register, you guessed it, another violation of the law.
Membership does have it's privileges. Access to libraries, scroll sanctoriums where you can prepare a scroll in peace. Access to labs and alchemists. In addition to all the amenities of an exclusive club. There is even a health plan of sorts. The guild does not tolerate its members in good standing being molested by thieves, corrupt nobels or such. Membership is exspensive, and there are multiple levels of membership depending on your level of spell casting ability.
The different guilds are highly competitive but not violent, unless there is some old personal grudge between the leaders of different guilds.
