How can mitochondrial DNA be used to trace ancestry?

mitochondrial is used for storage, so ancestral dna in cells can stay in mitochondria and being passed on

Human mitochondrial DNA is much smaller than human nuclear DNA ... For both of these reasons, the sequence of mitochondrial DNA stays the same over generations, and thus is a useful tool for looking at maternal ancestry. Scientists have found that the mitochondrial genome mutates 5--10 times faster than nuclear DNA.