is the boobs flash necessary for the plot to continue?
You can see it as something that helps to continuously establish the maturity of the story.
Did the film Die Hard need the scenes with the Naked Pin-up posters or porno-magazines; the scenes that gave it its R rating? Obviously not, but they helped set a mood and tone for the film. You can say the same for the tits in this scene.
Of course, not all nudity or 'mature' scenes do this - the random up skirt shots we often see being a good (or bad) example, as they rarely do anything theme wise, plot wise, tone wise, or do anything to serve the comic beyond just being fan service.