Saddle blankets do nothing to aid the fit of a saddle. They keep dirt, sweat and hair from affecting the saddle, nd may prevent small rubs from bumps/etc on the underside of the saddle (but a well-made saddle should have none).
Nowadays they have evolved to become an aesthetic accessory - They look good! Who doesn't love co-ordinating their gear or getting the smartest looking pad?
My saddle fits perfectly and doesn't need a pad. However, it has a felt underside and sweat would stiffen and stain the felt. I use only wool saddle cloths - They are even, don't get too hot or cold, aren't too thick, and look good :]