You don't really HAVE to have one, but it sure makes life alot easier on you. There have been a few threads about this recently here in this forum. If you are looking for the benefits and such of why it is such a good idea. Do a search for sumps and look at posts within the last month or two.
But, If you are looking to have an SPS tank, then I would say it is a requirement. It will give you more water volume for system stability as well as you will be able to run a larger in sump skimmer on the system. It also makes maintaining water levels easier as well as giving you a place to add top of water to and calcium/alk/mag supplements to allow time for dissolution prior to making it to the tank and possibly making corals angry
"Whatever you are, be a good one." Abraham Lincoln