This is not just the normal Big Five test. It includes the Aspects, for which each of the Big Five have two. For example, Agreeableness has the aspects Compassion and Politeness.
There are three main reasons to do it:
It's useful in understanding yourself better. These are deep personality traits that can be changed, though it takes a lot of time to do so. There are all kinds of interesting things these traits predict. For example, people low in Agreeableness are paid more than those who are high, so if you find that you're high, you can decide if you want to work on that to improve your career path.
It's the best of the best when it comes to personality assessments. It has been tested across cultures and generations, and it's been found to scientifically reliable and valid for decades now. Throw out that MBTI nonsense, which is neither scientifically reliable or valid. It uses clever language to make you think it's accurate. The Big Five is the real deal.
It can help you understand the current political climate. For example, high Neuroticism and Openness and low Conscientiousness and Politeness (aspect of Agreeableness) are pretty predictors or indicators that someone is politically liberal, with the inverse being true for conservatives. You may be surprised by what you find, and it will help you understand why you and others behave the way you do.
I recommend against doing a free version, because I can't vouch for them being the actual scales. I recommend www.understandingmyself.com, which only costs $10 and includes a detailed report based on your scores.