When the wet/dry filter started back in the mid 80's they where filters that contain a prefilter, to remove waste, a dry section, that contain a certain type of bacteria and a wet section, which contained another type of bacteria. They also were equip with denitr
ators, which removed nitrates.
As time went on the denitrators were removed, partly because some didn't work and partly to make the unit cheapier. Because these units now were producing nitrates most people got rid of the wet/dry sections, leaving an open section for additional water volume and more important water breakage. At this point they no longer were filters, as there was no filtration in them.
As the better skimmer came along, non-wooden air stone type, the sump was a good place to run them. And with the good water circulation it was a good place to put a heater too. Only when a filter
sock and refug was placed in the sump did it go back to being a filter.
Bottom line, if you take out all the filtration and bacteria sources from a wet/dry filter you're basically left with a sump which give you more water volume and good water breakage but it's no longer a filter.