What makes mitochondrial dna useful as a molecular clock?

It displays uniparental inheritance, since it is only inherited from the mother.

We know that because mitochondrial dna is passed from mother to offspring and cannot be messed with by genetic recombination we know that any changes in the dna must be caused by mutation. Because we know the number of mutations that happen in a lifetime we can use the amount of mutations that have happened between the two dna samples to find how much older one is than the other. Note: this will only work with members of its own family. Beyond that carbon dating is much more reliable