You harness the intangible but enormously valuable collective creativity, experience and motivation of a group that feels valued and that has a group identity (ala "The Alpha Project Team").

You take a large step toward either elevating a 'too conservative' Prime Mover project idea or you build a credible case for encouraging the Prime Mover to 're-think' (read 'forget') an unworkable, low yield and/or soon-to-be-obsolete project idea.

You transform a business need (commonly the Prime's reason for funding the project is to satisfy a profit or cost-related business need) into a specific outcome from which significantly more accurate and measurable ROI estimates can be made.

You find out how the project work will fit into each participant's existing priority matrix. In other words, you find out how likely they are to stop doing other tasks to devote enough time, when it's needed, for you to get what the project needs when it needs it.

You develop a possible schedule, that has more in common with the realities of the work environment that with potentially unrealistic Prime Mover desires.

You know whether you need your Prime to use his/her influence to 'adjust' organizational priorities so that your project participants are able to take time from other tasks and use that time to help you make tighter deadlines.

You know about and can plan around the full range of schedule and result-busting problems (vacations, shutdowns, unavailable talent, buggy software, etc., etc., etc.).

You begin the ongoing process of identifying what it is about the project that participants like or value. (Smart project managers inventory every possible project benefit they can for each individual project contributor so that, when the going gets rough, s/he can use benefits to maintain momentum).

You put yourself in the position of editing the good ideas of your project team rather than trying the much harder task of authoring a plan from beginning to end. (You also get a much better and much better accepted result).