What is the main reason for separating a group of individuals into a population?

Suffice it to recall the population criteria.

1) This is a group of individuals of the same species. If we are talking about individuals of different species, then this is no longer a population.

2) They live for a long time in the same territory, where most of their life cycle takes place. For this reason, a group of mosquito larvae in a puddle cannot be called a population.

3) They are relatively isolated from other similar groups (populations).

4) Population is an elementary unit of evolution, however, this is a long-term process, which cannot always be observed.

If the first three conditions are met, then such a group of individuals can be considered a population.