More Than Rough Brigands

Every so often you see someone point out that thieves' guilds could not possibly exist because they are so unrealistic. This is because the person saying this doesn't understand the idea of either how organized crime works or how secret organizations work. It doesn't help that most of these people are working with fantasy worlds; if you can accept the idea of fireballs, dragons, and talking swords as everyday problems, why is the idea of a thieves' guild such a problem? However, it may not be a bad idea to look at how these groups function and how they serve a practical purpose in most fantasy worlds.

What Exactly Are We Looking at Here

For our purposes, a "thieves' guild" is more than an organization where the local thieves and other criminals hang out; it operates like any other guild and offers training, general support, and an information source for its members. This means that they can seek out mentors, masters, and advice from other thieves. They can also find out who is corrupt and easy to bribe, floor plans for most buildings, and catch up on rumors. While they do offer legal assistance, it's given that the members will be spending some time in prison; their lawyers are more interested in fair trials and minimal punishment.

It should be noted that the medieval fantasy thieves' guild is also considered "legitimate" and usually has a seat at the table of any sort of town leadership. While its reputation isn't exactly pristine, it does provide a cap on crime and a limiter: By allowing some crime, the town keeps it under control (and the thieves' guild usually has control over most crime in the city, from the expected cutpurses and burglars to fences, prostitution, gambling, and some grift). Due to that, there is a certain degree of respect afforded the guild, especially when its skills are used against invaders, to spy for the town's leaders, or to deal with any number of problems.

Gatekeeping the Help

The biggest problem with running the guild is making sure you can trust everyone. In real life, this is why family members are usually chosen to join and afforded special status; at the very least, someone needs to vouch for the person. In a fantasy setting, this is a lot harder due to the constant influx of members, both locally and from other towns (you never know when you need a safe haven), so they may institute some sort of probationary period. This is also why the guild may enforce punishments that tend to be nastier than they would be if the person had been caught by the authorities.

Thus, there is some honor among thieves, enforced by some pretty hefty punishments, made only worse in magical realms.

Keeping the Tradition Alive 

Guilds are where beginning thieves learn their craft from master thieves (even though it's more likely journeyman-level thieves). It also provides a place where they can get advice and ask all sorts of questions, and there are plenty of books and manuals from famous thieves. This is also where they will obtain their education in the rules and traditions of the guild, making for a rounded education. The guild will also provide thieves new to the area an orientation of sorts that boils down to "honor the guild or pay the price." This allows the guild to maintain its history and reputation, such as it is, over the ages.

One's Place in the Greater Ecology

In the real world, organized crime fulfills three roles: they're the criminals, they operate certain businesses (usually for money laundering), and they act as a court of last appeal (in some cases even replacing the local authorities). In a fantasy realm, however, the thieves' guild usually only worries about the various stripes of crime. However, some specialists among their members can be used for spying, retrieving "lost" items, and helping adventurers get out of trouble. If you need a rogue, a spy, an actual criminal, or the crew for a heist the thieves' guild is the go-to group.

Thus, not only do thieves' guilds make a certain degree of sense in fantasy realms, but they are based on ideas presented by real-world organized crime. It's not that hard to create a realistic thieves' guild, and it can be sort of fun; after all, you don't need to be all rainbows and unicorns and engage your darker side, making it well worth the effort.