Preacher curl, simply put, doesn't allow you to cheat. The preacher curl bench (or machine) is explicitly designed for this exercise. The only difference between the free weight version and the machine is that on a freeweight preacher curl you use an EZ bar or a dumbbell, while the cable machine has weights pre-stacked for more comfort. Once you properly adjust the seat and the arm pad to fit your height the only thing left to do is to curl.
Preacher curl makes it almost impossible to do the exercise with poor form. And even more importantly, because of the position of your arms, and the pad beneath them, there will be no swinging or other types of cheating. Preacher curl is all about total control, all the way up, and all the way down. This puts more tension on the muscles, which will result in better gains.
Although it is possible not to "swing" when doing regular sitting and standing biceps curls, more likely than not you will start cheating once you get tired on those last few reps. It's just too hard to maintain perfect form under tension, especially if you do high reps. Additionally, when you do regular curls, you are more likely just to drop the weight to the bottom position, totally neglecting the eccentric part of the exercise. That will rip you off some serious muscle gains, as numerous studies show.
That is something the preacher curl tackles directly, and that is why it is one of the favorite pieces of equipment in every gym. It is also one of the smelliest, so make sure to use that towel, and clean the bench once you're finished!