yeah, for a SW
or a tropical tank (not sure which you have) you really do need a heater 99.999% of the time. While the lights may keep things warm during the day, overnight as things cool down (cooler with no sun, lights not running, etc) the water temp will fall. You really want to try to maintain a steady water temp, not let it go up and down very drastically.
That being said, if you live somewhere where it's just hot all the time and you don't run your A/C to keep things cold, then there are certain situations where you might not need it. But they're not very common.