As for the usefulness of the applications of macroevolution . . .
Use 1: determining the ancestral allele. We determine the ancestral allele at a locus by looking at closely related species (e.g. chimpanzee for humans). Knowing which allele is ancestral is very useful for a variety of reasons. For example, we use that information in searching for recent episodes of positive natural selection in the human genome. That, in turn, can tell us about important regions of the genome for various health-related traits. We used this approach to help identify candidate regions responsible for resistance to cholera (or for reduced severity) in
this paper. We're currently using similar approaches to look for genetic variants conferring resistance to Lassa fever in West Africa.
Use 2: estimating variation in mutation rates. We also compared humans to closely related species in
this paper, using the comparison to show that mutation rates don't stay the same over long stretches of genome. This enabled us to show, for the first time, that recombination in humans occurs primarily in "hotspots". This in turn meant it was possible to search for genetic risk factors for disease without sequencing the whole genome, and ultimately led to a massive program in whole-genome association studies, which have involved hundreds of thousands of patients and hundreds of millions of dollars.