Documentation is a marketing tool.
Modern Marketing/Sales departments realize the importance of a 'value-added product.'
A value-added product gives the customer more than the product itself. In the case of a good user manual or thorough, easily-navigated online help system, it facilitates customers using your product quickly to its full potential.
The faster and more efficiently customers use your product, the more your reputation improves within the industry. As your reputation improves, so do your sales.