It introduces formal considerations to what film commentary should be, in a simple way, because it focuses on a short bit that we easily grasp in our mind and one that is particularly significant to the whole film. As Jim says, the first image on the screen defines our symbolic connection with the meaning developped in the rest of the film.
Secondly, the collective participation opened to readers, so various viewers propose a new opening shot with their own personality, which blurs the institutional line between the unilateral critic's stance and the readers feedback.
Both aspects, formal criticism and collective criticism should be developped online more frequently in my opinion to take best advantage of online interaction and insights.
I especially like acquarello's essays at Strictly Film School because he pays particular attention to describe the opening sequence of each film. Thus we can picture the atmosphere of a film we haven't seen, and really get in the mood the auteur establishes which is important to best relate to his later film analysis.
And I like to do that as well when I review a film. This is a very good exercice. Giving a sneak-peak at the opening shot is more instructive and more pertinent than the random shuffle of a trailer montage.
At the reading of the analysis of Femme Fatale's opening shot, by Dennis Cozzalio and Jim Emerson, a film I haven't seen, I was inspired to try a little informal psychoanalytical reading of the first images :
The update of the femme fatale icon, as Jim points out, is also from B&W to color, an aesthetic leap, not only in fashion style. At the same time a nostalgic look back on the past, and a confrontation of two cinema ages, through the most possible violent comparison : superimposition. Giving the edge to the glossy color image of course. The old TV texture undermines even more the B&W image. It's like a comparative test : Color v. B&W
Then there is the multilayered apparition of the protagonist by progressive stages.
The TV set is also a mirror, physically, because De Palma uses it to film the face of his actress while she's turning her back to us. It is symbolically also because the viewer-voyeur, looks into the screen and wants to see his/her own reflection on the screen, meaning IN the movie. And this clever mise-en-scene achieves just that, the viewer and the star are side by side on the screen.
So that's a dialogue with the audience, a welknown feeling shared by a cinephile. The unidentified protagonist is like us, an audience within this fiction. Calling for a deeper identification with her.
Moving on to a deeper layer, we assume the protagonist's point of view. The girl watches a mythical icon and her reflection merges into her. The mirror image materializes this unconscious process of identification ("transference" in freudian term), like a ghost double of herself projected outside her body to rub off a little against the screen, and maybe become a movie star too. Which actually happens with the introduction of the real protagonist, pushing back Barbara Stanwyck to backdrop layer.
There is probably a more complex message about screen feminity nested there by De Palma. I don't know the narrative connection between Double Indemnity and Femme Fatale, so I can't figure the thematic purpose.
The reality check of the TV set and the subtitles continue to remove the viewer from the screen world. A trick meant to accentuate the reality of this hereby fiction (Femme Fatale), by contrast to another fiction film (Double Indemnity) turned into a mere video playing on TV and switched off abruptly. The nudity uncovered as the shot widens makes us more and more interested in the color diegesis.
Perverse manipulation to install the proverbial suspension of disbelief. Now we are ready to forget we are watching a movie, because the "movie" was on that TV, now what's happening is reality and we're in it!