"Well, do you really think Bernie will be able to pass anything if he's President?" NEWS FLASH: No democratic candidate is going to get anything passed if they are elected! Mitch McConnell will block everything if Dems don't win the Senate.
Because it still matters. 4 years of Sanders would mover the Overton Window ever farther to the Left & Republicans blocking very popular policies from going through will hurt them in the long run.