The Standard Greetings are a feature designed to help you not "re-invent the wheel" when you send messages.
Each of these greetings can be edited to what you want to say. For example, if you are planning a house party, you can set up a standard greeting with all of the information you want to send to attendees. Once you have filled in the information, click "Update" and the information will be saved.
You may see people talking about how they don't respond to "canned messages". The canned messages are those that are provided here by default. Creativity is the key, so if you have something witty to say to people to introduce yourself, put it here so you don't have to type it each time.