Figuring out the heating vs cooling of tanks and house has been a big reason for me to set this up.
For example, my 250gal tank is "heated" by 6 T5HO bulbs. That heat needs to go somewhere... In Winter, it is channeled into a small, unheated room - saving the need to heat that one. In Summer, it will just vent into the garage/outside.
Having the sensors makes it easier to determine how changes in venting or ambient temperatures affect the water temperatures. Two tanks in my office, both unheated, one running at 79 degrees, the other at 72. What's the difference? The hot tank uses higher light wattage, and better insulation, so it doesn't cool out at night when the room temps are down.
I have a 135 gal tank in an uninsulated garage, and only a 50W heater is needed to keep temps up. Why? Because the tank itself is insulated well. All plywood, styrofoam sheet covers up the front glass, tight fitting cover. Heating tanks is always easy. Cooling them is tricky.