While it is a different category that chromosomal DNA, it is still an essential part of mammalian life. None of use would exist in our current forms without mitochondria
human nuclear genome size (haploid): 3.1 billion bp
mitochondrial genome size: 16 000 bp
1 human nuclear genome per egg -> 3.1 billion bp nuclear DNA
100 000 mitochondria, each with 1-10 genomes per mitochondrion [1] -> 1.6-16 billion bp mitochondrial DNA
So the ratio of mitochondrial to nuclear DNA in human eggs is on the order of 0.5 to 5.