Sanded caulk is not meant to replace grout. You need grout on a floor between tiles because it is hard and durable. You use sanded caulk in places where grout would crack because it is not flexible but where you still need something that looks like grout.
Think of where your wall tiles meet your tub or where your backsplash meets your countertop. Those surfaces will move relative to each other and you need something flexible to seal between them.
Any kind of grout would not last long between the field of tiles on a floor, plus it would get really grungy really fast.
