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Sanders has a theory of change. You seize the executive with big, bold plans and rhetoric. Then use your pulpit to browbeat legislators into action or demonstrate their ineffectiveness; either converting or replacing individuals. If this is done, then you enact your plans.
It's hard and probably unlikely, but it's a genuine theory of change. It's what he did to successful ends in Burlington. A city is hardly a country yea but it's atleast a plan. What's HRC's plan to enact her legislative agenda? Negotiating is a dead art in a party system that makes sense.
Campaigns operate as networking centers. The Goldwater campaign in 64, though a profound defeat for the GOP, brought together conservative forces that stayed together and ultimately flipped the entire political axis of this country. Sanders has run the first genuine left wing campaign since probably Jesse Jackson in 88, and looks to have more success than even Jesse. That matters, getting people on lists matters, getting people involved matters, inroads with working class voters matter.
Most importantly sanders demonstrates that left economics is a winning issue within the Democratic Party. This is important. You show issues win and people start to campaign on them. Political fads matter too, even if sanders is just that. Most deride the Tea a party as a fad. That fad won 1000+ legislative seats in 4 years and shifted the entire window right by leaps and bounds.