I want to be have something I can use to reward desired behavior from the dog in training. The stronger the reward, or the more important the reward is to the dog the more effective it is for me.

So, I want to find the thing that is most important to the dog and use that for her reward. With puppies it is almost always a food treat. As the dog grows up I am building retrieve drive or tug drive with them. At some point the chance to retrieve or tug becomes more important to the dog than the treat.

That is when I transition over to using the other rewards.