Basically by bedding the action area of the rifle you keep the contact area totally locked together. In theory this helps prevent the action from moving in the stock and causing pressure points that could affect accuracy when shot. Same with barrel floating, you remove the stock from the equation so it doesn't touch the barrel and lets the barrel move any way it wants. Some rifles need to have contact but most modern bold actions like to have their barrel "free" and will perform the most consistantly that way. If the stock is touching the barrel it can bend it or move it. Also, if you use the sling during the shot you could theoretically move the barrel a tiny amount that would throw off the shot.