In the years doing the April Showers series it's become clear that there are basically four types of shower scenes in movies: sex scenes, sight gags, horror moments when characters are at their most vulnerable and emotional cleansing moments (that the body also gets scrubbed is just a bonus). Dragon Tattoo's shower sequence is clearly the latter type, after Lisbeth's brutal rape. David Fincher famously quipped pre-release that his movie had too much anal rape for Oscar. Oscar didn't mind so much nominating it for several Oscars.
Fincher's discussion of the sequence [after the jump. NSFW] on the commentary track is interesting.