One rationale I've seen is that uncontrolled spread and too many unvaccinated people overwhelms the hospital system, thus causing dramatic harm to people who need non-COVID related treatments. This dynamic played out in the US just a few weeks ago. That last wave had a larger proportion of hospitalized people who were younger and, in theory, less at risk because that group was less vaccinated as a whole.

The vaccines do reduce spread some, but they also keep people out of the hospitals.