What are the purposes of interpersonal communication?

No interaction between human beings lacks interpersonal communication. So, any purpose of a human is a purpose of interpersonal communication. There are good purposes and not-so-good, and bad. Many times, differing purposes will be at conflict with one another; for example, defending a country or preventing a war.

Most people will want to use effective communication for the good of themselves and society. This involves having shared goals and clear ways of communicating.

In general, any purpose that you can imagine would be a purpose of interpersonal communication.