Sumps are more often used in saltwater applications. In the SW world, most people don't even use a traditional filter. Instead, they use a sump with any or all of the following: live rock, sand, bioballs, macro algae, plus equipment. Very few use it in freshwater.
Part of the reason is that water is generally captured from an overflow meaning it comes off the top of the tank rather than from the bottom. This works with SW b/c saltwater has higher viscosity allowing a lot of the waste to rise to the top where in freshwater it falls to the bottom.
In order to make it work, you would either use a drilled tank or an overflow box. The water would drain into the sump and through a series of media. Then, a pump would move the water back to the main display.
The benefit is size. You can increase the overall water volume of your system by a great deal depending on the size of your sump. The downside is maintenance. Using a canister filter simply keeps everything moving through somewhat of a more "closed" system.
I have a 25g reef and a 40 gallon FW. I would never consider a sump for FW personally. Instead, I just use a high quality oversized filter that makes my life very easy (Eheim 2273)
