A Version control tool can be used to version any artefact, so it can be used for same purpose within testing as well.
A simple instance would be to version your testcases. Let us consider that the application you are working on is deployed in releases.
Each release would entail different functionality or could possibly be modification to an existing functionality. Incase you dont have a versioning tool or a Central repository that has self versioning, it is quite possible that a tester might pick up an outdated test case for execution.
But when you use a Versioning tool, you ensure -
1) Latest version of test cases are picked up for either making updates / changes based on Release notes.
2) Duplication is avoided easily since a test case once checked out can't be edited by any other individual
3) History tracking is maintained via the Versioning tool. You can easily identify the user and nature of change made
4) You ensure that the latest updated version of test cases are always used for Execution and not outdated ones.
There might be many more advantages of using a versioning tool for testing. Hope the above gets you started....