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).

an adaptation is a change or process by which an organism takes to be better suited for its environment. for example; migration is a behavioral adaptation, this is needed because birds are not built to last extremely cold climates. there are different adaptations because there are different species needing to adjust to different things, bears don't migrate; they hibernate.