If I understand your question, mitochondria are probably responsible for us existing at all. The partnership between our cells and theirs granted the energy required to become multi-cellular, and to start increasing the size of the genome.

It takes energy to copy a gene, and more genes require more energy. Oxygen is much more efficient energy source for cells than the alternatives.

There’s also a theory that having a separate set of genes that handled energy means the host cell didn’t need to worry about that bit, which allowed both us and our “powerhouse” to adapt more quickly and effectively to our environment.