Signings of new players for clubs

As usual in the pre-season there were many new signings of new players for clubs all around the world, some for clubs already host for over 30 players some of which is yet to grace the field. Why is it then these clubs see the need to be buying new players almost every transfer window is it just out of the need to put out new jerseys or is it that they/ the clubs have a genuine need for a new name in their squad.