Pretty much what Chris said. As well, are good for warm up and down too in training. Good cardio.

They are also a great tool for introducing new things, or changing habits or form. Say for eg. you want to break a bad habit down, and relearn a new technique, often it begins on the focus pads. Last year I had to completly relearn my left hook. So this meant also how it worked into various combinations. So the process begins on the pads, retraining those muscles, tendons, and over all memory there first with a coach overseeing it usually, then shadowboxing, then bringing that into sparring, to eventually have that change be effective when competing.

The pads are a important tool in boxing training, although often misused imo.