In the novel we don't see Darcy actually writing the letter, struggling with his thoughts, while in P&P2, we get to see that aspect of the whole letter-writing experience for Darcy. Also in P&P2, we hear the letter spoken in Darcy's voice. That is a very effective way for the viewers to change their opinions of the man.

And combining the letter narration with scenes that support the descriptions also helps the viewer: makes the letter less boring visually and gives us images to go along with the words. Also, this is the first time we see Georgiana in P&P2, I believe, and the image of this young, innocent-looking girl contrasted with the older, more experienced (and nefarious?) Wickham is very dramatic.