I've noticed that in order for my dogs to poop , they need to smell poop... why?

It may appear that way, but no, this isn't the reason a dog has a bowel movement. Dogs smell other dogs stools because that is what they do; the smell everything while walking, it all sends messages to them. That is why we call it 'pee-mail'. Next time your dog smells a pole, or fire hydrant, watch, you will see him raise his head and look one way or another.

Your dog is scenting. He is smelling the poop, than promptly putting his own scent in that area to stake his claim.

This is just an old behavior that followed out domestic pets from their wild ancestors.