All one has to do is ride up a hill with standard pedals and realize how much less force one can put on the pedals while staying in the seat. It has been verified that riding a bike requires a fith the engery of walking the same distance and this is largely due to the bike supporting a person's weight and no energy is used to decelerate as when walking. If you need to stand up in the pedals then you are using a lot more energy to move the bike forward.
For me there is also a safety consideration as having a foot slip off a pedal on a steep hill is often going to result in some injury. Even more of a concern when out riding in the rain. Even with sneakers I have used clip pedals to keep my shoes in place.