Bernie is trying to branch out to all minorities and this is a huge step for him and his campaign. This will help him win the minority vote and will lead to his Democratic nomination. There is no better person than Spike Lee to help Bernie Sanders accomplish this task.
Spike Lee was a good and clever choice for Bernie Sanders' ad because he appeals to a large demographic, which transitively enlarges Bernie Sanders' appeal. As an American film director, producer, writer, and actor, he is likable and relevant to many voters, which helps Sanders earn more votes that he may not have earned on his own.
Throughout the campaign, Sanders has had an issue connecting with minority voters, who overwhelmingly support Hillary Clinton. He also is seen as a "populist" candidate who wants to soak the rich. For this reason, Spike Lee was a great choice to make people listen who maybe wouldn't have listened to him before. I don't see it having much impact on the race, but it's a good political decision.
While Bernie Sanders has gained a lot of momentum in the race for the Democratic nomination, he still has a bit of a gap to close. Right now, Hilary Clinton is winning the black vote by a large margin. In many cases, Sanders is hurt by his lack of name recognition amongst blacks compared to Clinton. The right endorsement may help his cause a whole lot.