Old have accumulated wisdom, which young people typically don't yet possess, because they spent less time being alive. Such wisdom can benefit society greatly and thus old people have an increasingly bigger value to the species' survival in general. Individuals may not be able to measure or realise this value, but cultural evolution where various cultures compete against each other for longevity typically express this concept through various customs. For example, Chinese culture has been exceptionally stable for a long time, and you observe that they have a strong respect for ancestors and the elderly in general.

Old people in a modern society have the time and capacity to care for the young, because the parents are often working full time and have no meaningful time for their kids. Thus, indirectly protecting the old is protecting the young.

This is not a real benefit but it's a real effect. The young project themselves being old one day and thus want to create a society where the old is not discarded. It's long term thinking on the part of the young.

Specifically during a pandemic, keeping the R0 below 1 was an important consideration, no matter what the age distribution, because it helped slow the spread and thus allowed science and healthcare to catch up and avoid many deaths not related to Covid. At least that's the theory anyway. Execution is a different matter.