You absolutely will sell more things if you keep in touch with existing customers. This is a perfect use of your Customer Relationship Management (CRM) system! It's also great at tracking interactions with your perspective customers
after they have contacted you.
What your CRM doesn't do very well is explain why those customers bought or why those perspective customers contacted you. What
influenced those people -
Consider the scenario Steve White wrote about in this blog
post,
"A customer first discovers your dealership via a third-party auto listing site. Then they come back to your site later by searching your dealer name on Google and clicking on an AdWords listing, which perhaps was right above a free Google organic listing for your website. By default, in Google Analytics, that AdWords click took 100% credit for that visitor, assuming as a fact that it was incredibly valuable, when it may have had no impact at all on whether or not that sale was made."
That scenario was written about the limitations of Google Analytics, but the premise holds true; if you don't know what influenced the consumer, you are marketing in the dark.