He will get certain political ideas into the real mainstream, the same way Trump encouraged white supremacists to really make their voices heard. Except it will be things like universal healthcare: it's still seen only as a "radical idea of some of the Democratic candidates" but if someone is actually getting elected as President on the back of it the idea gains credibility even if the Republicans block it and fight it at every turn, as they are bound to do.
Don't forget that polls say universal healthcare as an idea actually has majority support among Americans but the accepted conventional wisdom still echoes the whole "radical idea" thing. Sanders getting elected would reinforce that. Eventually, it might get to a point where universal healthcare is seen as pitchfork insurance for the elite.