Sexual selection allows individual genes to survive instead of having to pass along the whole genome or none of it.
Scientists are confused at how sexual selection is so universal even though they only pass along half their genes per child.
If you pass along your whole genome, you also definitely pass along your whole genome. One fatal mutation kills the whole thing. This is especially important because occasionally you get lucky and survive for a few generations. (In the presence of welfare, several generations.) If you have a delayed-blast death aberration, you could potentially locally extirpate yourself, after you replace a majority of the population and then you all get wiped out.
By contrast, if you have sexual selection, about half your kids won't have the death-blast mutation. If, like many species, you have millions of kids, it's guaranteed to pass along basically your entire genome except the problematic gene, just by chance. The non-deadly "half" of your genes will be a different arrangement in each egg, covering all possible genes.
Many species give birth to millions of kids. "Only" passing on half => effectively half a million => way more than can possibly survive anyway. This is a non-cost. It's all benefit.
Put another way, on average two parents have two kids. If any population grew without bound it would have eaten the entire Earth already, and possibly moved onto eating the Sun. On average, extinction aside, populations are stable. Two kids, half each => 100% of the genome. One mitotic parent has on average one daughter => 100% of the genome.