Not sure where you are located geographically, but seasonal changes can make it difficult to keep your snake's ambient air temp at a steady 77º-80º throughout the year. I like to keep my home AC around 72º-75º in the Summer and 68º-70º in the Winter. In order to keep my snake's habitat at a constant 78º-80º, I use a Ceramic Heat Emitter (CHE) which needs to be regulated by the second Herpstat feature.

If your ambient air temps fluctuate widely throughout the year, you may need a CHE in which case you'll need to run two thermostats.