Film and literature are two completely different art forms. One of the best examples I've ever seen of this is from comparing the book Fahrenheit 451 with the movie. Both are quite different, but they tell the same story. (It's said that Ray Bradbury was involved to some extent in the making of the film.) There are scenes that work well in the film that wouldn't work in the book and vice versa.
Adapting any written work to film often includes many issues, such as finding ways to visually show on film what can be written about characters' thoughts, or dealing with issues of pacing.
Film is a visceral medium and literature is more of an intellectual medium. Film engages us in ways the printed word sometimes doesn't.
Blowing up the Burrow wasn't intended as much as a replacement of a scene in the book as it was a way to shock the audience (especially those who read the books and knew it didn't happen) and to make the point that, yes, these people are not only evil, but there is no place to hide from them. Throughout the stories, the Burrow is set up as a safe place. It's about the only place characters aren't hurt or threatened (other than the wedding, and that's outside the building).
The trope is known as The Worf Effect (Warning: TVTropes link - go there and you'll lose an hour of your life following endless links!) If you're telling a story that takes place on the Enterprise-D and you want to establish from the start that your alien is tough, you have Worf go up against him from the start and lose. (Another example in Star Trek of this same trope is in the Star Trek: Deep Space Nine episode The Jem'Hadar where they bring in a Galaxy Class ship to beat the Jem'Hadar, and it gets blown to pieces almost immediately. The only purpose was to tell us, "This enemy is deadly and powerful.")
The only two reasons for this in Harry Potter and the Half Blood Prince were shock value and to make the point emotionally that no place was safe and that the enemy was quite lethal.