As an end-user, it's not usually not needed to buy 3D Mark, the basic free one is fine. If you were doing reviews of systems and video cards, then you would want to purchase the full version. Of if you really wanted to customize the tests to stress your system and see how it runs with higher settings.
Your system is a high end one, there is no need to run a benchmark, it's fast enough for any game. Only time you would want to run it is if you are comparing systems or having an issue with games running slowly to find out if it's the games or the system.