Traceability matrix is prepared in order to cross check the test cases designed against each product requirement.
Why we use the Traceability Matrix?
If you prepared your test cases completely & client changes the requirement ,obviously you will need to change your test cases to meet this new requirement. But ,your test case suite is very large and it is very difficult to trace the test cases affected by the test cases. So if requirement were numbered & we can put the test cases id's to corresponding requirement, It would have been very easy to track the test cases.
*Product Requirement Id is PR001
* Test Cases- TC001, TC002, TC003, TC004
Here PR001 is a requirement in the software requirement specification Document(SRS).
TC001, TC002 , TC003 & TC004 are the test cases which are prepared for that particular requirement.
If any defect found during the execution of test cases, Defects ids will be entered in the defect columns in the test cases spread sheet & for track the that particular test scenario, you can entered the test case ids & Prod req number in bug report description.So it will helps to tracking the defect against requirements.