Many people hire a guide to help them "learn how to do it" for their next solo trip. This is one of the most rewarding sorts of trips for the Guide. The guide is able to teach someone how to have fun, be safe, and enjoy themselves. This allows the client to go a little above their comfort level, with a safety net of the experienced guide.
Others hire a guide because they want to enjoy the wilderness, but do not have all the strength or knowledge they need to spend time in the environment by themselves. Here the guide is a bit more of a hired safety tutor/pack animal... not the best of all worlds, but given a grateful client, an OK thing.
There are also those who hire a guide because they don't want to carry or clean or cook. They seldom hire the same guide twice. They are frequently as dissatisfied with their guide as their guide is with them.
Some outfitters offer guide training programs. These can be excellent hands-on learning experiences, and they can make wonderful vacations. Even if participants don't intend to work as guides, guide training programs and classes can greatly increase a paddler's knowledge and confidence, and can offer many tips on improving one's techniques. Before you sign up, though, contact the outfitter; you should learn about the requirements (fitness level? paddling experience? equipment?), and you should also ask whether the outfitter welcomes "vacation" guide trainees.