"Why is it important?" I'll give you seven good reasons...
1. It is perceived as addictive. Under the Controlled Substances Act of 1970, marijuana is classified as a Schedule I drug on the basis that is has "a high potential for abuse." What does this mean?It means that the perception is that people get on marijuana, they get hooked and become "potheads," and it begins to dominate their lives. This unquestionably happens in some cases. But it also happens in the case of alcohol--and alcohol is perfectly legal. In order to fight this argument for prohibition, legalization advocates need to make the argument that marijuana is not as addictive as government sources claim.
2. It has "no accepted medical use." Marijuana seems to yield considerable medical benefits for many Americans with ailments ranging from glaucoma to cancer, but these benefits have not been accepted well enough, on a national level. Medical use of marijuana remains a serious national controversy. In order to fight the argument that marijuana has no medical use, legalization advocates need to highlight the effects it has had on the lives of people who have used the drug for medical reasons.
3. It has been historically linked with narcotics, such as heroin. Early antidrug laws were written to regulate narcotics--opium and its derivatives, such as heroin and morphine. Marijuana, though not a narcotic, was described as such--along with cocaine. The association stuck, and there is now a vast gulf in the American consciousness between "normal" recreational drugs, such as alcohol, caffeine, and nicotine, and "abnormal" recreational drugs, such as heroin, cocaine, and methamphetamine. Marijuana is generally associated with the latter category, which is why it can be convincingly portrayed as a "gateway drug."
4. It is associated with unfashionable lifestyles. Marijuana is often thought of as a drug for hippies and losers. Since it's hard to feel enthusiastic about the prospects of enabling people to become hippies and losers, imposing criminal sanctions for marijuana possession functions as a form of communal "tough love."
5. It was once associated with oppressed ethnic groups. The intense anti-marijuana movement of the 1930s dovetailed nicely with the intense anti-Chicano movement of the 1930s. Marijuana was associated with Mexican Americans, and a ban on marijuana was seen as a way of discouraging Mexican-American subcultures from developing. Today, thanks in large part to the very public popularity of marijuana among whites during the 1960s and 1970s, marijuana is no longer seen as what one might call an ethnic drug--but the groundwork for the anti-marijuana movement was laid down at a time when marijuana was seen as an encroachment on the U.S. majority-white culture.
6. Inertia is a powerful force in public policy. If something has been banned for only a short period of time, then the ban is seen as unstable. If something has been banned for a long time, however, then the ban--no matter how ill-conceived it might be--tends to go unenforced long before it is actually taken off the books.
Take the ban on sodomy, for example. It hasn't really been enforced in any serious way since the 18th century, but most states technically banned same-sex sexual intercourse until the Supreme Court ruled such bans unconstitutional in Lawrence v. Texas (2003). People tend to be comfortable with the status quo--and the status quo, for nearly a century, has been a literal or de facto federal ban on marijuana.
7. Advocates for marijuana legalization rarely present an appealing case. To hear some advocates of marijuana legalization say it, the drug cures diseases while it promotes creativity, open-mindedness, moral progression, and a closer relationship with God and/or the cosmos. That sounds incredibly foolish, particularly when the public image of a marijuana user is, again, that of a loser who risks arrest and imprisonment so that he or she can artificially invoke an endorphin release.
A much better argument for marijuana legalization, from my vantage point, would go more like this: "It makes some people happy, and it doesn't seem to be any more dangerous than alcohol. Do we really want to go around putting people in prison and destroying their lives over this?"