While not essential, different harnesses can be used for tracking and protection depending on handler preference.
For protection training, a sturdy harness makes holding the dog much easier since you are holding him near his centre of mass. Dogs which are large and strong, small dogs, unco-ordinated pups and dogs who drop the sleeve when there is pressure on the collar all benefit from working in a harness. Generally though, it is a matter of personal preference. The harness should be snug so the dog cannot wriggle out of it, but you should be able to slide your hand underneath it. It should have a strong D ring on the top of the dog's back to attach the lines.
Bottcher harnesses are specialist tracking harnesses and are allowed in trials. The design is supposed to help keep the dog's head down. It is also useful for dogs who tend to rush while tracking. The harness can also be put on upside-down across the dogs back to help with stand training.