The plan is often the first introduction an Investor has to you. It shows the Investor how you think, and demonstrates to some degree your experience, and it indicates who you know and what they think of you (the person who introduced you to the investor). However, until it's read it's just paper and ink.

Second, the plan puts you in the driver's seat. In it you have described how you're going to build a successful company, and have outlined measurable benchmarks upon which you want to be judged. At the end of the day it's your vision, your milestones, your talent that will drive the investor's investment not vice versa. However, if you can't do what you said you'd do, be prepared to step aside or shut down.

Third, the business plan acts as a pact between you and the Investor. Spoken words are easily forgotten, but the plan is what is remembered and acted upon.

Fourth, all the research you have done for your business plan is the basis for your Due Diligence Preparation. By their nature, Business Plans are brief and are organized to "sell" the opportunity. Hopefully, it will raise questions that the Investor wants you to answer face to face.

For further reference:
Business Planning Guidelines/ Submission Guidelines
Submit Your Business Plan