WHY do we need citation styles?
WHY are there different types of citation guides?
A.P.A. (American Psychological Association) was established for psychological publications, but through the years, has been adapted by other disciplines. There are several others--including M.L.A. (Modern Language Association) and Chicago style. Always check with your professor to see what style is used for your class.
WHY are there many editions?
The manual are periodically updated to reflect changes in how society obtains and uses information. "Back in the day" there were typewriters and no internet. Now we have internet with many sources and ways to use them.