Okay... lets say that you are using the words "good boy" to mark the exactly correct moment of a new behavior you are teaching, and then give him a treat.
You are using the "good boy" to mark the behavior... and the treat is the reward (reinforcer).
Generally people create this association almost by accident. Its the dogs association... praise, then reward comes.
The Primary Reinforcer is the reward... you are teaching the dog to respond to a secondary stimulus that 'means' reward to the dog, so that the timing of a reward itself, is less important to shaping the behavior. This means the click (or 'yes') is a Secondary Reinforcer.
Since most dogs can sit, jump, and turn around twice in the time it takes to say "good boy", it is easier to give more precise information to the dog if you choose a shorter word (I use "yes") or a click, to identify with more precision to the dog. Timing is important.
This method of communication can make it easy (with an experienced dog) to teach them a new behavior, or behavior chain, at a distance, without touching them. When the timing of the treat itself would make it very difficult to give the dog specific meaningfull information.
It also makes it easier for the dog who is so glued to the fact that you have the treat... they cant think. The dog is working for the click. Its the only way to get the treat.
I like that it teaches my dogs to work FOR something (food, me) rather than to avoid something (correction, me). They try harder.
To me... corrections are for creating inhibition, or avoidance, of a behavior. Like jumping, pulling on leash, etc. When I teach obedience behaviors I want to teach the dog to think about what I want him to DO...
not what to aviod. A leash has no use for me to teach a precision heel... it is something I add later as a distraction.
If I tell you to walk next to me... and you had to figure out where or how, wouldnt you be able to do exactly what I wanted easier and faster if I told you exactly how and where... or if I only told you what NOT to do when you made a mistake? Which would make you better at your job? Or make you give up?
Its also a fun way to train. I love making up new, creative things to teach them, just to see if I can.