Technically if the saddle fits right you shouldn't need any kind of pad. So, basically it's just to keep the saddle clean.
Gillian is right. The only use for a saddle pad is to keep the saddle clean from the horses body. Nothing more, nothing less.
Marketing companies like to make money, so they come out with schpeels that saddle pads correct and protect the horses back, and many buy into it and spend their hard earned money - putting them in the pockets of the companies who shovel them out.
All a saddle pad is meant to do, is just keep the dirt and sweat from ruining the leather of the saddle panels.