Studies have proven that goals that are committed to paper are more likely to come to fruition than those that are not.  So, it is true with a project as well. 

There are many benefits to creating a Project Management Plan including:

• Clearly defining roles and responsibilities of individuals involved with the project
• Clearly defining processes and activities to be addresses within the project
• Increasing the likelihood of completion on-time and within budget
• Ensuring understanding of what has been agreed upon by all parties involved
• Outlining how project activities will be managed (scope, cost/budget, quality, etc.)
• Serving as a reference guide