Why is marijuana sacred among some Hindus such as Naga Sadhu?
Shiva plays a role in this but does marijuana aid in obtaining moksha or does it have any spiritual benefits?
Marijuana or any other drug is not considered sacred by Hindus. There might be some exceptions where people justify it. The usual argument these people give in favour of drugs is a misleading argument that Shiva consumed Bhaang. Hence, it is okay for them to do so too. This argument has no base in it:
Q. Gurudev, we sing devotional songs in praise of Lord Shiva and Lord Krishna. But back home, the people there consume Bhaang (a type of Indian narcotic incorrectly thought to please Lord Shiva) while they sing praises of Lord Shiva. How should one understand all this?
Sri Sri Ravi Shankar: See, if Lord Shiva consumed Bhaang, then he also consumed the poison that came from churning the cosmic ocean. So should we also drink poison first, and then see if we stay alive to drink Bhaang? Do not misunderstand all these things. Lord Shiva never consumed Bhaang. The essential point to understand is that the Shiva Tattva (the most fundamental all-pervading element in creation) resides in each and every one. An intoxicant like Bhaang too cannot do anything to Lord Shiva’s bliss, because he is above and beyond all such trivial influences. But chanting his name can have a positive influence on us and elevate us.
Excerpts from a Q&A: http://celebrating-silence-of-life.blogspot.in/2015/08/why-good-people-suffer.html
Patanjali's Yoga Aphorisms (IV. 1.) says: "The Siddhis (powers) are attained by birth, chemical means, power of words, mortification, or concentration."
Ganja (and some other drugs) can open up - temporarily - some chakras in some people. But when you do, it is only temporary and results in a fall just as dramatic. The best way to open these chakras and control it is through meditation and concentration - not through drugs.
When opened temporarily, a person can sometimes access certain powers temporarily. Accessing these Siddhis, however, is a diversion from Self-Realization and can easily put you back farther.
Nagas will smoke it for several reasons, one for the reason above, another as it can put off the feeling of hunger when you are starving, and third, they are just not that sincere as sadhus and enjoy being drugged.