There is no doubt that the more product knowledge you have the more sales you can write. Following up with clients who already purchased a product or service from you, updating them with more information about that service or product, will make them realize how much you care about their business. This will give them good enough reason to refer you to people they know to purchase from you.
Besides, the more product knowledge you have, the better you can qualify a customers need.
Here is a great quote from Reid Hoffman
One of the challenges in networking is everybody thinks it's making cold calls to strangers. Actually, it's the people who already have strong trust relationships with you, who know you're dedicated, smart, a team player, who can help you.