The common description of Brazilian Jiu-Jitsu is that it is a grappling martial art involving the use of throws, takedowns, and ground control positions leading to chokes and joint locks, utilizing principles of leverage to allow a smaller, weaker person to subdue a larger, more powerful opponent. In reality, it is far more than that.
Research has shown that more than 90% of fights and attacks end with both attacker and defender on the ground. This is incredibly uncomfortable for humans as we are bi-pedal (we walk on two legs) and spend most of our time upright. It is the natural human response that when we are forced to fight on the ground, our brains basically 'freak out'.
BJJ allows us the opportunity to train ourselves to not only be calm in such an unnatural situation, ut to thrive. This also gives us an 'unfair advantage' as we will be comfortable fighting on the ground while our attackers won't be!