When you eat your own dogfood (or rather, feed it to your own dogs), to demonstrate confidence in your product. You can experience things as your users do, which can help give you a better understanding of how people use your product and the issues they may find.
That was one of the unfortunate realities when developing Riak at Basho: while we used Riak, we didn’t need its massive scalability, so it was hard to find the problems that customers would eventually stumble upon.