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.