Why is it important to know the average mutation rate when using DNA comparisons to determine how long ago lineages split?

mutation rates can be used to design molecular clocks in order to estimate divergence times among species  

Explanation:

In evolutionary biology, the substitution rates (also known as mutation rates) are used for the development of 'molecular clocks' which allow estimating the divergence times among species or among taxonomic groups. Molecular clocks can be designed by estimating the divergence rate of nucleotide sequences (either DNA or RNA) and amino acid (protein) sequences. These substitution rates can also be used to track how quickly viruses evolve (for example, in a pandemic situation).

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