Well... for some kinds of meditation, weed helps a LOT... i used to meditate and also try to astral project far before start to smoke. I had a very small sucess in astral projecting, and learned at least how to relax my body, even if keeping the mind quiet were an unattainable thing at that times.
After i started to smoke, i noted that the indica strains are very helpful to silencing the mind and also astral projecting. Ive read that weed is in fact used in the preparation of Bhang, used by buddhist monks, and im sure it helps their meditaiton. Indica makes the mind too slow and stoned to think, so its easy to make it be quiet.
Also, it makes us less aware of the physical body (confortably numb... couchlocked... yeah... :stoned, and so its easier to leave it (which is the astral projeciton).