Because this is THE moment. The moment Robbie (James McAvoy) falls deep in lust with Cecilia (Keira Knightley). That lust would lead to that filthy letter. The letter leads to that hot encounter in the library. That's what Briony (Saoirse Ronan) sees and misunderstands leading to the unravelling of all three lives.
And because Keira Knightly looks damn good wet.