Peanut butter is sticky, so the dog won't

let it stay in mouth for long time and swallows it down fast.  That's a plus.

Also, it's handy than meatballs & icecream balls ( can stay in room temp), and has a strong tasty smell, so it's attractive to dogs.

All of these have the same basic idea, one can alternate the different kind of food if the dog gets bored. :))