A sump is not a filter, it is a place to put stuff, to get that stuff out of the main tank, and out of view. This might include filter media, but not necessarily.
If you just want to get heaters out of view, filter socks, protein skimmers, or tubes and the like. It is also a great place to send new water from water changes if temp is not similar to the main tank, or its chemistry is slightly different.
But I agree a 10 gal is a bit small, one of the best reasons for a sump, is to get equipment that you do maintenance on, in a roomy, user friendly spot.
It can also be a place to put a fish that needs convalescence, or fry, or a refugium for plants, or invertebrates, if they don't survive in the main tank.
I use some sumps for planrs and as refugiums, without any equipment, and small tanks can work well for that.
