Need of Test Plan and Test Cases for testing

Test Plan identifies the test activities and also defines the objective, approach, and schedule of intended testing activities, while Test Cases comprises of the test procedure, test conditions and expected result.

Writing a test plan early in the project lifecycle, and having it peer reviewed by Development in general helps reduce the workload later in the project lifecycle. This allows testers to quickly and unambiguously complete the majority of the testing required, which will provide more time for "Ad Hoc", "Real World", and "User Scenario" testing of the product.

Test case is comprised of a test condition, an expected result, and a procedure for performing the test case. These can be performed either in combination with other test cases or in isolation.

A testcase is the difference between saying that something seems to be working okay and proving that a
set of specific tasks is known to be working correctly.

Test Plan talks about "What has to be tested" while Test Cases talk about "How to test" because of which both these documents are of equal importance in testing.