it also adds additional water volume (for more stable water parameters), more surface area (for more aeration) and more flow (detritus settles in there and can easily be vacuumed out when doing water changes). I have a lot of room for filtration in my sump: filter sock, filter pad, protein skimmer and carbon when needed. i also have additional live rock in there, i have 60 pounds of live rock total in my system and 60 pounds of sand in a 50 gallon display (with sump, i have 60 gallons total system water volume).
you can't cut corners in this hobby, it will come back to bite you later when your system starts to mature. I spent over two and a half months researching, planning and building my system before I even put water in it. And i've had a reef tank before! Let me tell you, it makes a huge difference.
edit: i've tried to link a good article about starting a reef tank but it will not let me link outside forums...