What's the significance of marijuana?

Using it, right? Mj has been linked to a 3-fold increase in schizophrenia. It's use has been linked to decreased brain function, & linked to shrinkage of the thalamus, an important brain structure that controls or modulates neural activity. All in all, a wolf in sheep's clothing. In many cases it is definitely a gateway drug introducing acceptance to other drug experimentation & possibly addictions. It's illegal.
Nothing good! Marijuana is a hallucinogen which effects your brain in a number of negative ways, including causing psychotic episodes. And, yes, it can lead to abuse of other drugs.