I. Statistical applications: Biologists are fond of performing statistical studies, looking for meaningful correlations between different parts of animals' anatomies. For instance, one might look at the ratio of the length and depth of birds' beaks vs. the length of their tarsals to see if larger birds have proportionately deeper beaks. Such studies require that all observations be made from the same underlying population. However, when samples are being taken not from a single pool but from groups of populations nested in a heirarchical phylogeny, this assumption is violated.
Fortunately, biologists have developed a statistical technique for adjusting data to account for the phylogeny of the taxa sampled, called independent contrasts. By applying it, meaningful correlation studies can be performed. Without it, they would be meaningless and misleading.