网上的理由是“Without it, any moisture that works its way to the Hot Mop would have no where to go. Over time the moisture would build up bacteria and MOLD, that eventually come up through the grout joints.”但是moisture到了斜的Hot Mop还是和水平的Hot Mop一样还是要停留那里，只是斜的Hot Mop会帮助moisture往低处（drain area）聚集。