"Why in the world would an adult want to buy a tricycle", you might ask. Anyone whose ridden one can tell you why ... They are safer, they are more comfortable and they are better for your body than a bicycle is. Besides, especially if you buy a recumbent adult tricycle, they look really cool, too!
As we get older, we're usually not quite as coordinated as we were when we were younger. Our muscles are usually not quite as strong relative to our body weight and while falling off a bike at any age really hurts, adults usually end up with far more serious injuries than children do when they take a tumble off of a bike.
Obviously, three wheels are a heck of a lot more stable than two are. That's why small children usually start off with a tricycle. An adult recumbent tricycle rides much lower to the ground, which means it has an even lower center of gravity than a traditional upright tricycle. That makes it even harder to tip over. What's more, in the extremely unlikely event of a fall, you don't have far to fall. Less distance and force means it is far less likely that bones will be broken!
More than anything, adult tricycles are more comfortable than bicycles are. Adult trikes typically have wider seats and a great many of them have seat backs, which greatly increase your comfort and help alleviate back strain. If you go with a recumbent adult tricycle, you'll also reduce the strain on your wrists and shoulders.