This is a great question! The reason is that your broker holds the listing contract and you, as their agent, are representing them. Just like all the other agents in your broker’s office. Since all of you have access to your broker’s listing contracts none of you are sharing privileged information by discussing those contracts among yourselves.
In order to promote your broker’s listing contracts with anyone outside of your brokerage, whether by email, fliers, signage, phone calls, social media, or skywriting you need to allow for all agents to have the opportunity to learn of the listing. So you are required to have a listing entry in the MLS. For SFARMLS. Your listing entry can be either an Active or a Coming Soon listing; but it must be in the MLS prior to any discussions held with someone who is not a member of your brokerage.