Formal English writing enables people to use words that are appropriate to their purpose and intended audience. The use of "dear" as a greeting in formal letters and emails is polite without being overly familiar, and doesn't cause offense. In business communications, this customary salutation is followed by a colon, while a comma is used after the salutation in personal letters and emails.
Informal writing is used in communications with friends and coworkers. It would seem strange, after all, if you addressed your best friend as "Dear Mrs. Smith" in a letter or email. However, addressing her by her first name prefixed by "Dear" followed by a comma, is appropriately intimate. "Greetings" is an acceptable salutation in a letter or email to friends. It could also be used when sending a Christmas or birthday card. You could, for example, write "festive greetings " before your name on a Christmas card. An informal group email to friend or coworkers might begin with "Greetings everyone," rather than the more formal "Dear all."
"Dear" followed by the recipient's name and title, such as Mr. of Ms. is the customary salutation for business letters because these are more formal than personal letters. "Dear Sir or Madam" is used when the recipient's name is unknown. It is acceptable to use "Dear" followed by a first name in a more informal business letter or if you are acquainted with the addressee.
It's best to stick with formal English guidelines when writing to people you don't know. "Greetings" would look out of place and breach etiquette in a resume cover letter or a letter to your bank manager, for example. Sticking to the rules conveys professionalism and demonstrates familiarity with the rules of written English.