I see posts with put the probe between the mat and tub/tank. Why?

i jave always put the probe where the important parts are, i.e. inside the tank where the snakes actually are. The mat could be any temp, what matters is where the snakes are.

Caveat: my dart frog tanks had the probe between mat and tank. Solely bevause the little buggers can escape through the eye of a needle. Was a huge pain tweaking that all the time.