Well, sumps can hold the ectra water volume which in my opinion is the first and best reason.Secondly, you can hide all your equipment in there. Heaters, reactors, skimmer etc. If you get a sump with filter socks its just matter of swapping out a filter sock every few days and just keeping the sump in good working order. A sump with a refugium is even more useful when you go to a reef tank.
When using a canister filter if you dont maintain it and clean it out frequently you will have a smallnitrate factory on your hands. Plus depending on what you run in the fluval will dictate how often you have to replace media. If you run carbon you have to change it out like every two weeks, and make sure the sponges used don't become clogged or full of gunk.
I was to say the least, VERY intimidated by a sump. Like crazy scared to try it. BUT it worked out. Mostly from input on this forum. I had some poeple literally hand hold while I set it up. BUT I am glad to say it's been up a few months and I've had 0 issues with it.
Here are some links that might help
Refugiums,fuges or refuge
Berlin method of filtration
Protein skimming - is it right for you?
Hope they help a little. if you need more info just ask away