Brazilian Jiu Jitsu; at its core is a Self defence martial art where in a One vs One scenario a much smaller person has the possibility to control even the largest of aggressors. Brazilian Jiu Jitsu is a science, it's a set of principles that uses leverage, weight and control to subdue the untrained.
Thanks to the growing popularity of Brazilian Jiu Jitsu (and partly due to the exposure of the UFC) BJJ became so famous it was created into its own sport. Just like all other sports there are rules that need to be followed.
But without the aspect of strikes and defending yourself for self defence BJJ took another turn its its growing popularity, it is an ever evolving sport due to the nature of the body and mind.