I've never understood it either. There are a bunch of settings in Slic3r like that. I always specify numeric values for speeds, line widths and layer thickness. I want to know exactly what the slicer is going to use. I also don't use the "default" or "auto" options for anything. In the past, Slic3r has tried to extrude 0.2 mm wide lines with my printer's 0.4 mm diameter nozzle.
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]