The biggest reason to support his candidacy is that it will force Hillary Clinton to tack left now and stay more on the left side of the spectrum until well into 2016. I am not exactly a Hillary fan. Don't get me wrong, I think she is vastly superior than any of the members of the present Republican clown car who have either declared or are about to. But I find her untrustworthy, a little too close to Wall Street, and like her husband a little too keen on triangulating her positions to what sounds best to get elected.
Ever since the 2012 election it has been obvious that she is going to run and there has been an aura of inevitability surrounding her march to the Democratic candidacy. Having at least one serious challenger is good for her campaign in that it will actually make her organization have to work in the spring of next year and therefore find out earlier if there are any problems that need fixing before general.
The other thing is that while she is truly center-left in her politics I think she's too center and not much left and having to respond to an avowed socialist like Sanders will require her to start articulating more progressive positions especially in spheres of taxation, income inequality and entitlement programs. To quote President Harry Truman:
I've seen it happen time after time. When the Democratic candidate allows himself to be put on the defensive and starts apologizing for the New Deal and the fair Deal, and says he really doesn't believe in them, he is sure to lose. The people don't want a phony Democrat. If it's a choice between a genuine Republican, and a Republican in Democratic clothing, the people will choose the genuine article, every time; that is, they will take a Republican before they will a phony Democrat, and I don't want any phony Democratic candidates in this campaign.
After eight years of a Democratic president, if Clinton goes out and campaigns too far to the center that she sounds like a fake Republican the electorate will decide they may as well have a real Republican in that case. Then gods help us all.
That's why I like the idea of Bernie Sanders running for president. He will make sure we have a truly Democratic candidate on the ballot in November next year.