You can keep equipment like heaters and pumps out of sight.
You will have constant surface extraction which is the best IMO
You will have no hard water lines from evaporation in the tank as evap will happen in the sump.The tank must always be filled to the same level to overflow through drain pipe so it never changes.
You also have the opportunity for much more media be it mechanicals or bio..
In general my opinion is they are the best filter but take up a good amount of room in the stand[or wherever you place it]. The sump for my 180g in my living room is in my basement....I drilled a hole through my floor in the stand to achieve this !