the slicer does what it says, it cuts the map into slices and writes each slice in a BMP file. People can use these drawings later to get a precise contour of the map. But one problem, the slicer doesn't apply any textures, the drawings are black&white.
http://hpb-bot.bots-united.com/bsp_tool.html
actually I've always wondered how mappers do their overview bitmaps. I'm not sure it's the slicer they use.