Not a "thermometer" or anything of the sort. It's a complete monitoring package -- we sell them, or at last a similar one (I don't know which brand was involved, probably a knock-off). They're really handy. They monitor temperature, oxygen, pH, ORP, and a bunch of other things, can control pumps, lights, etc., do automated tasks, and even provide live streaming video of your tank. They're lifesavers with large, high end tanks.
They act as a fail safe for bad heaters, for one thing -- your heater gets stuck on, they turn it off when it reaches 77. They'll turn the chiller on, send an alert, etc. They'll tell you when a fish has died in the tank.
When you're talking a large, expensive reef tank, the $600 you'll pay for this system is worth it, and a lot of freshwater aquarists could learn a thing or two, too.