There is of course, another reason; they're good if cooked properly. So how do you rotisserie cook a chicken properly? First of all, remember that rotisserie cooking is a form of roasting. To roast something simply means putting it in the oven uncovered. Putting a bird on the grill with the lid down is roasting. What the rotisserie adds is even cooking. Because the bird is constantly in a state rotation, it will cook more evenly and the turning keeps juices moving and not dripping. A rotisserie chicken will be moister than one that has simply been roasted.
Presumably, the final reason for the popularity of rotisserie, or roast chicken, is the fat content. By placing chicken on a rotisserie spit where fat can drain away and not in a fryer full of oil, there is a drastic reduction in fat.
The first step to rotisserie cooking a chicken is having some kind of rotisserie and knowing how to use it. This is where that user manual comes in handy. Once you have the basics of how to use the particular piece of rotisserie equipment for your grill, it's time to get the chicken. It's best to start with a fresh, not frozen whole chicken. It really does make a difference.
The next thing to consider is the seasoning. Marinades, dry rubs and bastes will all work well with rotisserie chicken. Traditionally, we think of basting meat on the spit, but that can lead to burning and requires more attention. Of course, you may still want to baste during cooking to keep the bird moist, but a good marinade will do more towards a juicy bird than applying sauces during cooking. When applying marinades or rubs to your chicken, do your best to get it under the skin and to the meat. Coating the skin will help to make it tasty, but won't do a lot for the meat underneath.
For the specifics of grilling your chicken on the rotisserie see my article on Rotisserie Chicken - Step by Step.