It's a good thing to have for an aquarium to allow you to not only keep the tank from falling below a set temperature if colder weather at night or some other cause lowers the water temperature but a heater allows you to raise the temperature if you are treating fish for some sicknesses, like Ich.
Just to add a bit on to FLBettaCouple's comments...while fish can tolerate a fairly wide range of temperatures, what they can not tolerate as well is wide variations in temperature over short periods of time. It can stress them and lower their resistance to disease. Consequently, as FLBettaCouple pointed out, having a heater can allow you to put some limits on the bottom range of your tank's temperature so that you don't have to worry about large temperature swings. Just a thought. The range you mention as normal for your tank does not sound that bad, and would likely be fine. My concern would be more along the lines of what happens during the winter, or on an abnormally cool night.