You help the Prime Mover develop a detailed description of what s/he wants, needs, won't tolerate and could live without so s/he isn't surprised by a project result that doesn't jibe with his/her vision of the project outcome.
You are able to gauge the project's priority in terms of the money and resources that the Prime Movers think the project should require and the amount of both that they are willing to spend in a 'worst case' scenario.
You know where the real DEADline (as in you wish you were dead) lives on the Prime's calendar and where the "I'm beginning to get nervous" deadline hangs out.
You know what needed and available resources the Prime has direct control over and what either you or s/he must go out after in order to make the project possible.
You have a good idea of the connection, if any, between the project and (a) the company's overall strategy, (b) the annual departmental goals and (c) the Prime's performance (read bonus-impacting) goals. Of course, these connections have some bearing on your goals as well. These connections give you a good idea of the consequences that will be forthcoming if you bring the project in on time and budget and if you don't.
You know who will be using the result of your project and who will be benefiting from that use (many times these are two different people and/or groups) which allows you to gather information about what the users will expect and what their management will inspect.
And you know whether the participants you'll be seeking out are likely to stand in line to be part of your project, hide under their desks when they see you or confidently beg off because they know, due to any one of several factors (eg, your Prime's reputation, the history of the project idea, the departments that are competing for available resources, etc.) that your project is a non-starter .