Why is the Hardy-Weinberg law useful?
First, let's define a couple of terms. An allele is a particular form of a certain gene. Various genes in our DNA can be found in different forms, or alleles. A phenotype is the visible set of traits an organism displays based on the alleles present in its DNA.
The Hardy-Weinberg law states that in the absence of outside influences such as mutation or genetic drift, the allele and genotype frequencies will remain constant over time as the population of organisms being studied reproduces over time. In other words, the frequency of alleles and genotypes produced in successive generations can be reliably predicted with each successive generation under idealized settings. This utilizes the Punnett squares often seen when talking about genetic frequencies.
As you know, however, ideal conditions are rarely found in nature. Populations of organisms evolve genetically over time and produce new and different shifts in allele frequencies and observed phenotypes, preferring some and discarding others (evolution). So monitoring the actual genetic frequencies of a population and comparing it to the idealized outcome predicted by Hardy-Weinberg allows geneticists to determine if a population is evolving or not.
The Hardy-Weinberg's law is important primarily because it describes the situation in which there is no evolution, and thus it provides a theoretical baseline for measuring evolutionary change. The equilibrium tendency tends to conserve gains which have been made in the past and also to avoid too rapid changes.
Hardy-Weinberg's law provides a situation where the genes in the population have reached the equilibrium and the gene pool is constant. In such a case there will be no change and no evolution. But it has been observed in nature that over a long period of time equilibrium is disturbed and changes do occur on account of several forces.
Rastogi, V.B.(2000) : Organic Evolution