Writing, in advance, about your upcoming research serves several purposes. First, it allows you and your mentor to begin carving out a project that is appropriate for your level of expertise and for the time allotted. Second, it forces you to think more deeply about the content of your work and provides a foundation for your project ahead. And, finally, it gives you an opportunity to fine-tune your technical writing skills.
Depending on your program, you will either need to submit a proposal with your application or complete a project plan after you've been accepted but before you start your project.