A guild served a few purposes. These are in no particular order.

Guilds ensured that those who belonged to the guild was qualified to do whatever work he represented he could. There were long apprenticeship programs that ensured that a new worker knew what he needed to know.

Guilds controlled rates of pay. A guild member would only perform X work for Y amount of money.

Guilds controlled access to employment. By regulating the training program, they could ensure that there was never a glut of workers who would drive wages down. OTOH, they could restrict entry to training in order to ensure higher wages.

Many guilds developed fraternal ritual stuff, sort of akin to the Masons. This led to a stronger guild, reinforcing all of the other functions of the guild.

Guilds functioned sort of like a trade school does today. For example, think of DeVry. They don't do the 'high-minded' liberal education -- they focus on training folks to be able to do whatever they're certified in. An employer sees a degree from DeVry and knows that the graduate most likely has the particular skills the employer desires. A guild membership card functioned in much the same way. A member of the printer's guild could show a card or other proof of membership and the print shop owner (most likely also a member of the guild,) would know that this person knew how to set type or bind books, or whatever.

Guild membership allowed the member to travel, knowing that his membership card would likely enable him to secure employment. This is the root of the term 'journeyman.' An apprentice would graduate, become a member, travel (the journey,) then create a masterwork which would enable that worker to become a master at his trade. He could then train new apprentices and start the cycle again.

My specialization is late 19th-early 20th C, so I've likely left off a couple things, but this is the gist.

Current organizations that function like guilds are academia, the legal profession, and the medical profession. All of these are (nearly) self-regulating and control access to the profession.

For a fun read on the subject, see W Rorabough, The Craft Apprentice. The book deals heavily with the printer's guild --> union in the US, starting with the late colonial period.