Nature and purpose
Fundamentally, all communication is about making another person understand something which you have to say, which could be a request/ demand, an explanation etc.. .
So, in designing the delivery mechanism, one should not only consider the message to be delivered, but also the other person’s ability to receive that message.
Even by silently observing or by being absent, you are participating.
Establishing good memories in others
People choose between memories of experiences, rather than experiences themselves. So, the ending is extremely important in any interaction. Giving gifts at the end of formal occasions is a good idea.
Research communication
Described elsewhere.