The wet belt process takes off any surface imperfections in the base material that occur mainly from rock or other non-snow articles the skis may run over. The composition of the base coating is such that rocks or whatever not only cause a gouge in the ski, but push the material up on each side of the actual scrape.
After the wet belt process you have a completely flat ski bottom. Minor scrapes will be planed down by the sander but any really deep scoring will need further attention.
Jack and Dan's Tip: When skiing, remember to be aware that although they don't look like that big, even little pebbles can dig into the ski bases.