I’ve shown empirically that mtDNA is selected for, in particular by male humans when selecting female mates. See A New Model of Computational Genomics, in particular, Section 5. The data suggests quite plainly that males are selecting females on the basis of having overall similar mtDNA genomes. I didn’t explain how it is that males or females would know this, though I now think the answer is, that mtDNA is deeply connected to overall health. Therefore, it makes perfect sense for a species to develop a means of communicating information about mtDNA, since doing so, would allow for better mating decisions, and therefore, healthier offspring. I don’t know what the specific mechanism is, but I’d wager mtDNA actually does impact appearance in some subtle way that informs attraction between mates. This would in turn allow males to select females on the basis of having a similar overall mtDNA genome.