Why? What are the benefits? Obliged
Most of the time, to drill a new firing pin hole so the firing pin protrusion is a tight fit so primer metal won't flow into the gap. Also to improve firing pin strike accuracy especially if the firing pin hole is slightly off center of the primer when striking. In a few instances, the bolt face will be rough or damaged by escaping gases from damaged or blown primers and needs to be either faced off or, in severe cases, bushing the bolt face. I'm pretty sure I haven't hit all the reasons but I think these are the main reasons for bolt bushing...
