Question: Why is marijuana sacred among some Hindus such as [Naga Sadhu](https://en.wikipedia.org/wiki/Dashanami_Sampradaya#Naga_Sadhus)?

Shiva plays a role in this but does marijuana aid in obtaining moksha or does it have any spiritual benefits?

Answer: 
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.
Yes. Marijuana is mentioned in the Vedas:

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> पञ्च राज्यानि वीरुधां सोमश्रेष्ठानि ब्रूमः। दर्भो **भङ्गो** यवः सह ते नो मुञ्चन्त्व् अंहसः॥ (Atharva Veda 11.8.15)
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Hindi meaning

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> “Patra, kand, phalphul aur mulatmak panch rajyoun se yukta aushdhiyon main Somlata sarvasherstha hai. Darbha, **Bhang**, Jau aur Dhan ye sabhi humse stut hokar **hamare dushkarmo ko katne main samarth hon.**”
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It means marijuana will destroy our bad karma