Why do organisms have a common name and a scientific name?

The reason that organisms have common and scientific names is because there has to be a universal name for all organisms in theory.

Common names can be many different things, dependent on the area of the world you live in, Scientific Names are part of a series called binomial nomenclature which is a universal way to name all organisms. For example, a goose will be called anser anser across the world, then it changes some when you go into breeds, but for the most part, you can get by with that name.