You would use them to create a particular frequency response, with different slopes in different regions. In practice though, it might not be so much that you would want to design a set of low-pass filters for some particular use... rather, the common situation is that you have some circuit that's made up of different components, to do whatever thing you're doing, but the components act like filters and you have to figure out what the frequency response is (and whether or not it's okay for your application).