It Provides a Focal Point for the Test and a Milestone

Without the test plan, details get fuzzy and ambiguous, especially under time pressure. The test plan forces you to approach the job of testing systematically, and it reminds the development team of the impending dates. Having said all that, it is perfectly acceptable, and highly probable, that the test plan will be developed in stages as you gradually understand more of the test objectives and talk to the people who will be involved. Projects are dynamic, and the best laid plans will change as you begin to approach testing.