That sounds like group selectionism. In what circumstance would this be deleterious to the propagation of that individual's genes?
The less diverse the population, the less likely timely mutations will help protect it. Incidentally, our modern society is pretty diverse, especially as we protect those who couldn't otherwise survive without modern medicine.
https://www.lesswrong.com/posts/QsMJQSFj7WfoTMNgW/the-traged...
Among a single species, with a 50/50 split of a genetically diverse and homogeneous population, if you eliminate 10% of the former and 75% of the latter, the split becomes 80/20. If that distinction stemmed from genetic factors, you have selected for diversity.
Genetic diversity isn't only advantageous to the population. It's advantageous for the individuals that carry it, depending on the circumstances. Therefore it can easily be selected for.
There doesn't have to be a use for an evolutionary trait. It can be an utterly useless feature. The fact that it persists in this fleeting moment we co-exist with it in this particular million years, only shows that a feature hasn't been rendered extinct. Yet.
But of course, I believe that biologists can construct a cladistic tree that can calibrate how long a particular trait might have been around.
https://www.scientificamerican.com/article/what-is-the-funct...
And another suspected purpose:
https://www.webmd.com/digestive-disorders/news/20071012/appe...
You can never really know the reason with evolution because every plausible effect is in play and part of the optimization process. Having said that, one reason for adults to die may be to conserve resources (food) for the young in times/areas of scarcity. Another may be to prevent excess breeding by an individual, which may reduce genetic diversity. There are probably other viable reasons as well.