An email should begin with a greeting. If the recipient is a close friend or relative, an opening like "Hey Mom" or "Hello Larry" is fine. For a business email, a more formal greeting is appropriate.
The purpose of a greeting is to provide an opening for your message. The expectation of a greeting has long been rooted in the etiquette of letter writing, and taking the time to include a greeting shows that the writer is polite enough to acknowledge the recipient before launching into the message. After the greeting, move into the purpose of the email before finishing with a polite closing.