Hillary Clinton needs the support of Bernie Sanders because she performs so poorly with young voters. Clinton needs the endorsement of Sanders as a way to appeal to young voters that have been weary of her. If Sanders does not support Clinton in the general election many of his supporters may not vote for her. Therefore, yes, Bernie Sanders' support does matter.
Hilary really can't refuse him on this even in the event she wins the nomination. She will not win without the supporters of Bernie Sanders, many of whom are currently unwilling to vote for her in the general. She will desperately need a Bernie Sanders endorsement. I hope Bernie takes it all the way to the presidency, but if he doesn't win the nomination his influence is still going to be felt quite strongly in this presidential race.
Yes, I think that Bernie Sanders' support of Hillary Clinton will matter because Clinton struggled with defeating Sanders and Sanders proved to be a challenging opponent. His endorsement will help unite the Democratic party and bring millions of disillusioned voters to her camp against Donald Trump. Sanders' support is very important in helping her secure a victory in November.
It seems like the media and many of the voters have already reached a decision about who goes on to the primary elections this time around and it will not be Sanders. Whether he supports Clinton or not the election will continue to be Trump vs. Clinton. He has been lagging in t polls recently and most Sanders supporters have gone over to the Clinton side.