I think the benefits are obvious. Lets say you have 10 bugs and they are prioritized in order from 1 to 10. Bugs with the priority 2, 3, and 6 have been given

I think the benefits are obvious. Lets say you have 10 bugs and they are prioritized in order from 1 to 10. Bugs with the priority 2, 3, and 6 have been given a severity of P0 (a ship-stopper, as the original post described it). Bugs 1, 4, 5, and 7-10 are not considered P0 bugs. Given those priorities and their relative severity, that means the team is committing to deliver, at least, bugs 1-6 prior to the next release. That makes severity a pretty useful dimension to consider on its own and not just as consideration when determining priority.