Writing manuals and blog posts, as well as creating screencasts, forces you to explain your design to other people. This helps you find issues with your design: if something is hard to explain, it may be hard to use.
People read manuals when they’re already unhappy, so manuals should fix the problem, not make people even more unhappy.
Press releases and blog posts help you think about your product’s goals and put the focus on the problems you want to solve. Can you explain your product in a single sentence? If not, maybe you’re trying to do too much or you’re not solving a specific problem.