The lower part of the front fender, if it extends low enough, blocks spray that would otherwise hit bottom bracket, chainrings, and feet. My Longboards keep all three clean and dry in wet conditions. (They might get wet from rain falling, but that's clean water, and doesn't leave the gritty residue found on parts exposed to spray.)

Edit to add: Grant Peterson might have explained it best in Rivendell Bike's SKS Longboard description:SKS Longboards have very little ground clearance. I would absolutely not recommend installing them without the quick release stay attachments. (Mine have released twice in use, likely preventing two accidents.)