Priority seating also tends to be nearer the ends of the carriage, so that it's easier for people with a mobility impairment to access.
Note, just because you attempt to book priority seating using XC's website, there's no guarantee that you'll actually get a reservation for it. (Booking via the XC Assistance team or this website's 'seat picker' is more likely to be successful).
Also, if you need one for genuine 'disability' reasons, be aware that which seats are priority differs between XC 220/221s and the ex-Avanti 221s. As these are used interchangeably, it's worth checking the seating plans at
https://www.crosscountrytrains.co.uk/on-board-with-crosscountry/our-trains to make sure the seats you're booking are priority on both types!
I don't know what the official position is if a person with no accessibility needs has reserved a priority seat but an elderly/disabled person without a reservation has a greater need for one, although I know what I'd consider the 'moral' position to be.