The first reason is fairly obvious. Most people looking for a project to join have the same few generic questions regarding things such as payment, choice of technology and the goals of the project. The various sections of the template provide the majority of that information, saving people the need to ask and allowing people to quickly glance over the sections that most interest them.
The second reason can be somewhat less obvious however. Having to fill out the template forces people to think about those details of their project, and (although this doesn't seem terribly effective) it is hoped that in at least some cases will encourage people to do a little more preparation before forming a team. The majority of sections are things that any well thought out project should be able to answer without any problems, and apart from those headings marked as optional it should probably raise warning-flags if you're having trouble coming up with answers to any of them.
Thirdly, a poorly filled out template can often be a quick indicator to experienced users browsing the threads that a project probably doesn't have good chances.
Look at for an entry in the next few days on potential alterations to the current template.
If we look at the hobbiest/new indie population looking for help, you're going to find the vast majority of people have no idea what they're actually going to be doing, or even what they're looking for.
How many times have we seen the stereotypical 'idea guy' looking for 'programmers', it's all too common and I think community leaders and other experienced folk should step forward and say something to the effect of:
"I can see you're new to this, typically, 'this' is how things are done."
It is of course 'easier' and at some levels more enjoyable to mock the person; however this normally comes from the typically defensive response from the unskilled and unaware.
What should be applauded (and I see it now more on the HW forum) is the poster who is willing to listen and potentially accept that they are going the wrong way about something, for the benefit of their potential project; that is what will get a game done, after you stir in a whole bunch of staying power.
The template is a great tool for weeding these folks out and has made HW a far more respectable place; there is no harm in cutting the volume of posts if the volume cut is worthless.