Software Developers are linear thinkers
Software Developers can think through a long sequence of logical steps in developing a software application. Getting a software to work needs a lot of time spent in debugging and fixing code. Developers are constantly looking for a working path through multiple scenarios available at each functional state of the application. They inherently assume, if the previous action or step works, then then can build the next action. So a software product is developed through a series of linear thinking and writing of code by developers.
Software Testers are lateral thinkers
Software Testers can think across the functionality available in a software product and come up with ways to use the product, which the developer has not thought about or imagined. So the value for the developer in having skilled testers is in getting quick feedback of the product either negative or positive. This feedback can help the developer catch issues with functionality that should have been there or are missing in the product. Developers thrive on quick feed back in building the product. If one has skilled testers and they can give positive feed back to developers early on in the development cycle, lots of pain and expense associated with shipping or releasing a buggy product can be avoided. Developers can delight your customers with a high quality and functional software product working with skilled software testers.
99tests is building a community of skilled testers, who can help developers/entrepreneurs in getting quick and high quality feedback on the quality of software products and enabling applications to be released that are much more stable and delightful to use.