"So why was this recommended, and defacto mandated, for that age group, again?"
Because by preventing cases or even just reducing the virus load, it decreases the likelihood of spreading the virus to others.
Approximately 100% of people ended up getting COVID, the overwhelming majority - repeatedly. So once again these were claims that, while at least reasonable on the surface, were made without any evidence in support of them and turned out to be, if not false, then misleading.
So people now tend to change the goalposts - okay it didn't stop the spread or stop people from getting it at all, but helped spread out the spread - flatten the curve, and reduce the impact on hospitals. But again that also seems completely false. Here [1] are the data on cases in the US. By August 2021 the wide majority of Americans had taken one of the shots. The biggest surge, by an overwhelmingly large margin, would come on January 2022 where we went from a former peak of ~250k to a new peak of more than 900k daily cases.
So now the goal posts get shifted yet again. Okay it didn't stop the spread and it didn't flatten the curve, but it reduced the rate of severe cases. This one is a bit trickier. It's superficially true, yet subject to extreme biasing. If you look at the overall outcomes of people admitted to hospital by vaccination status, unvaccinated individuals did often have worse outcomes. But there's a rather huge bias - people inclined to vaccinate for COVID are also the type more predisposed to seek healthcare earlier, whereas those disinclined to vaccinate tend to be less inclined to seek healthcare unless it's critical. This bias (one amongst many) was repeatedly listed in the limits of various studies, but people just ignored this (and them) even though it's a major factor. There was never any study (to my knowledge at least) that tried to compensate for these biases.
