Controlling the temperature of a reef tank is very important, and most hobby grade heaters can be accurate enough on their own. One of the biggest keys to success in this hobby is making sure that everything has a fallback or redundancy with heaters being one of the most important. Typical heaters will last a year or so until one day they will fail and either get stuck on, or stuck off. By adding a temperature controller like a ReefKeeper to monitor and control your heaters power source, the chance of failure is almost none.
By connecting your aquariums heater to a ReefKeeper Controller, and then taking a few minutes to program the controller, you can have the ReefKeeper monitor the temperature, and if it goes over your set point, it will turn cut the power off to the outlet turning your heater off. Then when the temperature goes below the set point, the outlet will turn back on again.
Another nice feature of the ReefKeeper is its ability to have alarms set if parameters go out of range. The controller can alert you with audible beeps, visual flashing, and even email you if you have the NET module. With a correctly programmed ReefKeeper, you can have a fully temperature controlled reef tank, with your lights, fans, and heaters all working in conjunction, keeping the waters temp super stable.