A lot of movies are pitched at children (or adults) with a mental age of 8. Some folks in the audience haven't read the book, didn't pay attention to the first 6 scenes of the movie, and haven't decided yet if Harry Potter is supposed to be the good guy or not. So, the director has to "help these viewers" along. How to do so?
Have the Death Eaters show up at someone's house and set the whole thing on fire for no reason at all. This establishes that the Weasleys are good people being victimized by bad people. And it establishes that the Death Eaters are (wait for it...) actually bad guys.
I'm sure I'm over-stating this, but I swear some directors really try too hard to make sure everyone "gets it".
I thought it was more to show how dangerous these times are. Even the Weasleys, powerful wizards affiliated with The Order, can be and were violated in their home.
Although, I really don't understand either how they were able to get to The Burrow itself and around the enchantments. I try not to think about that much.