The same reason to vaccinate everyone else: to minimize the opportunity for a more virulent strain to emerge - one against which current vaccines are ineffective.
That won’t really make much of a difference in new strains emerging.
https://fortune.com/2021/07/14/covid-vaccine-tracker-update-...
> Given the flu kills more kids than covid
Emphasis mine.
https://www.google.com/amp/s/www.newsweek.com/leaky-vaccines...
Selection pressures and non sterilizing vaccines are what causes more virulent strains to emerge.