I have read that ordinarily your brain holds on to a lot of junk, an example would be things that happen in your day both good and bad and things that go much deeper than that.
Cannabis is supposed to help your brain sort thru the good and bad, relevant and useless and help relive some of the mess going on up there. Now this is particularly helpful with PTSD because it helps your brain forget the hurtful or harmful memories and make room for new stuff.
It's also considered a neuroprotectant and has been benficial in helping people with dementia etc..