Brake Discs rely on good contact between the brake Pads and the Brake Disc. As a Disc gets older it can get scored and wear unevenly. They also tend to warp a little due to constantly getting hot and cold during normal operation.
When the Pads are replaced they have a perfect surface and if the old Disc is installed without machining back to a perfectly flat surface, the pad only to grips on the high points so brake efficiency can be poor.
To recondition a brake Disc, it is put in a Brake Lathe and the braking surface is Skimmed back to perfectly Flat.
Brake Disc in Lathe being Skimmed. You can see the ridges on the disc and in the centre where the cut has been started you can see a shiny area between the 2 grey rings(Cut area). This is a low spot which had to be machined out with a second cut.