A thermostat, turns the heat on or off, or controls the amount of power going to a heating element at any given time, to maintain proper levels of heat output.
A thermostat is a MUST for any Under tank heater ( UTH )/Heating Pad, as they can and will burn your snake if not properly regulated.
I personally use a Herpstat for my large rack enclosures, and i use Zoo Med 500R's for two other racks. I find the 500R's to be decent t-stats for the price and both have worked for me for 2-3 years without issue. ( some people are dead set against them, and i am not at all. ) They are cheap enough for a single enclosure, but capable of controlling larger rack systems under 500 watts.