There are several benefits to user experience design, and I'll list some of them. Before I begin, a quick note: user experience design to me encompasses EVERYTHING, from UI design to customer support processes to retail store design and packaging. So when I speak of "user experience" I'm thinking about any touch point a user can have with a company.
Increased customer satisfaction. The better experience you create for your customers, the happier they will. And the opposite is also true: the worse experience you provide to your customers they will become more and more frustrated with what you are providing them… And they will be far less likely to recommend your offering to friends and families. And dissatisfied customers call to complain, which means when you provide a good experience you also have…
Reduced cost of ownership and support. If you produce a product (hardware or software) that has an easy-to-learn (and easy-to-use) design, you will have to support that product less. Good design also reduces your total "cost of ownership", in that you will need less documentation, a smaller support staff, and less salespeople. Which brings us to…
Increased sales. Happy users share their happiness with their circle of friends and family. They also review your offering online. Providing a good experience helps build positive word of mouth, and increases sales. It also often results in increased customer loyalty and therefore repeat business.
Good karma. OK, this one is not really a measurable benefit, but I think that if you create something that helps people do something well and makes thier lives better then you will benefit from that effort. Case in point: One of my first major successes in user experience design was the creation of a streamlined process to sign up for electronic bills. This process resulted in a huge "hockey stick" uplift in adoption, which in turn meant a lot less trees were destroyed to print paper bills. My company did well, it had a positive environmental impact, and the success helped my career. Win, win, and win.