Majeed,

By this we get variences within and between breeds, that will ultimatly used for selection and development of superior genotypes.

When considering al of the breeds as one sample we can get a value of the total variance across al individuals across all breeds, But we do not know how much of the total variance is due to differences among the breeds themselves and the variation within each breed across the individuals of those breeds.

An example among humans is that the variation across races is not significant but that within Africans and within Swedes, there is a great deal of genetic variation. The conclusion difference among races cannot explain any part of the variability among humans as a whole. Each of these populations varies within that population so genetic variability is not at all explained by race.

Similarly if we had many populations of cows and calculated the total variance of all individuals we would know that cows vary across the world. But we would need to separate the components of variance due to population (geographic location) from those among individuals.

In practical terms for humans we know that genetic differences across the globe are a negligible component among all humans. In other words variation is distributed evenly across the globe so we know that intelligence, for example, does not vary among races although we know that it varies among individuals in each race. Intelligence cannot be explained by genetic differences among races.

In parallel, suppose the opposite is true of cattle, which is understandable because each population of people may breed for attributes that are particular to the environment. Now suppose that the cattle do not vary genetically within each population - that each cow in a particular population is pretty much identical genetically. But suppose that each population is truly genetically different from the others. Now suppose that a particular population is too uniform genetically so that they cannot adapt to new local variations in the environment. What is a breeder to do? In this case introducing cattle from other populations could well give the genetic variation that can respond to environmental changes. If we have a source of variation that we can add to a particular population in peril, then we might save that population.

If the cows are more different within each population then adding genetic material from other populations would not help.

Suppose Antarctic populations of penguins are uniformly genetically similar. If we would like penguins to survive in Antarctica, should we introduce genetic variation from tropical Galapagos penguins?.

In genetic divetsity study of any organisms, it will enable us to select the novel genes which are the desired trait of interest and produce what we would like to...

One reason is that it can provide an indication about previous inbreeding and genetic drift that has occured due to the founder effect or bottlenecks. Also, as a general rule, the more variance the more raw material that can be used for selective breeding programs.