The purpose while swimming; The palm of our hand should be creating the largest surface area with the fingers and thumb. If we compare our hand to a table tennis racket; The larger the paddle, the greater the pulling force that can be applied with each arm pull.
Contrary to popular belief, if we combine the thumb and other fingers of our hand, we will not have created a racket that will absorb the water in the best rate! To explain this better, we need to examine a little fluid dynamics:
As a solid object moves through the water, the layer of water next to it also moves. The sticky layer of water increases the surface area of the object. The force of the water will slightly flatten the skin on the fingers and create a webbed effect.