First, I have never worked on a project in my (computer) career where the customer was not oversold on what the product would do or the timeframe, causing the engineering staff to work 80 hours a week for months on end while the salesman, job completed, went home to sleep on his gold-plated feather bed.

Second, selling products and services is (obviously) an important part of the economy, but the act of selling itself is worthless. It doesn't produce anything, and it often extends the life of products that are substandard, while retarding the life of products that should overtake them. In other words, if your product doesn't sell itself, it's extraneous.

(Note that I'm excluding advertising from sales; it can be hard to make your product and its features known. And that's the problem faced here.)