You might think that once you kick off the development process, most of the work should be done by developers that you’ve hired. While that's true to some extent you still have to spend plenty of time road mapping, planning, and managing the process.
Compared to product management, project management is more tactical. It is as vital as keeping the strategic vision for your product. Even if your development team consists of only one person, you still need to make sure that all requirements are understood and deadlines are met.