While replacing a brake disc you tend to notice the state of the mounting bolts.
Do you always have to replace these too? No, but wouldn't it be a shame if your brand new brake disc fails to work properly, because it is held in place with bad bolts? Rust, gravelly roads, the weather, road salt, they all have an effect on the looks and performance of your hardware. And let's not forget, loosening those old bolts can be a pain as well.