the main thing is it increases your water volume which = less likely chance something can go bad quickly, second you hide most of the clutter in your tank like heaters, pumps for the skimmer, the HOB filter. in a sump you can also have a fuge which you can load with rock rubble for added biological filteration and with some chaeto to help control nitrates. you can also use it as a spot to house troublesome fish say if you have a damsel and its picking on your clowns you can catch it and house it in the sump until you figure out what to do with it, its an easy spot to put an Auto top off, its a good spot to hide probes and such if you have reefkeepers, you can keep your fuge light on opposite schedule from the main display to help maintain PH levels. there are tons of benefits to having one!

i have a 29g with a 10g sump and it fits everything in it, i also have a 5.5g frag tank with no sump and a 1g with no sump because they are to small