Usually they are also responsible for ensuring that authors under contract deliver typescripts to specification and on time.
Furthermore, in textbook companies, this role is responsible for managing title revisions, signing new authors, and ensuring products are profitable.
Recently there has been some attempt in the UK to articulate and establish professional standards.
Most commissioning editors work up to the post through experience in other roles e.g. Editorial Assistant.
There is little consensus on what makes good commissioning editors, though most people in the industry would agree that the following are desirable and important: The standard work on the subject is Gill Davies' Book Commissioning and Acquisition (Routledge).