The destruction of the Weasley house added no plot element and it was not in the book.
The Weasley home was basically supposed to be the safest place for Harry (aside from Hogwarts). Why did the film writers decide it was okay to destroy it?
There are quite a few instances in the Harry Potter movie series of loyalty to the books being sacrificed to make more audience-and-money-grabbing action scenes. That's pretty much the entire plot of the last one. And remember the first trial in the fourth movie?
From the IMDB FAQ page for Harry Potter and the Half Blood Prince:
In a new scene created for the movie, the Burrow is attacked by Bellatrix Lestrange and Fenrir Greyback during the Christmas break from Hogwarts. The scene was added in because the film's middle act would otherwise have had no action, and it would also show that there are no safe places anymore. The scene also helps develop Harry and Ginny's relationship a bit more because, during the scene, Harry runs after Bellatrix and Ginny rushes to help him. Also, Lupin and Tonks are featured in this scene, which establishes that they are a couple. Finally, the scene ends in a rather melancholic note given that the Burrow holds practically everything the Weasleys ever worked for, and since they were already quite impoverished, it adds to the darker tone of the movie.