It Serves as the Main Communication Vehicle

The test plan serves as the main communication vehicle among the main designer and developer, the test moderator, and the rest of the team. The test plan is the document that all involved members of the development team, as well as management (if it is interested and involved), should review in order

to understand how the test will proceed and to see whether their particular needs are being met You use it to get buy-in and feedback from other members to ensure that everyone agrees on what will transpire. Because projects are dynamic and change from day to day and from week to week, you do not want someone to say at the end of the test that his or her particular agenda was not addressed. 

Especially when your organization is first starting to test, everyone who is directly affected by the test results should review the test plan. This makes good business sense and political sense as well.