My former colleague Miller Hogan, who retired from TRG a few years ago, once blogged about the topic of sales and marketing interaction. He provided a simple but insightful chart about the relationship between the two functions. His blog is worth a read so I encourage you to take a look.
He concludes with a note about the customer buying process. This is so important to companies big and small. In short, where your customer is in the process determines what you need to be doing. Early on? Awareness is crucial; tactics from advertising and direct mail to internet communications and PR are commonly used. Later on, when the buyer is aware and understands more or less what you do, it’s essential to solve a problem. So the sales function will probably need to provide a specific solution to what a customer requires.
In short, understanding your customer means more than just understanding their wants and needs; it also means you know where they are in their journey.