A lot more mask removal here, but as Schafer points out, it does allow Spidey to emote!

The cynical answer is that actors like to have their own faces on the big screen. From a practical standpoint it is much easier to express emotions with facial expressions than with body language or dialog. And it is oddly disconnecting to listen to dialog being delivered by an immobile, static face.

The better answer is that despite the title, Spider-Man isn’t about the costumed persona, but about Peter Parker. Spider-Man is an icon, both in the real world and in the fictional New York — people care about him in the way that they care about the Empire State Building or the Mets or FDNY. Peter Parker is a human being that we can relate to, like a particular athlete or a relative who is a firefighter. We are more connected to Peter Parker than we are to Spider-Man.

Removing the mask not only connects us to the character better (and the mask is only for the benefit of the fictional citizens of fictional New York — the audience already knows who he is under the mask), it also humanizes the character within the fictional world.

On the elevated train, Peter removes the mask when one of the eyepieces is obscured. This allows the audience to see both the mental struggle of trying to figure out what to do about hundreds of tons of metal speeding toward a sudden end, AND the physical struggle of his efforts. With the mask in place, all we see is Spider-Man acting decisively — we don’t get to see the doubt and the frustration and the pain.

The absence of the mask also allows the passengers on the train to see Spider-Man in an extremely vulnerable state. They comment on how young he looks, about how similar he is to someone they know. They become the caretakers of his secret; by protecting his identity they show just how much New Yorkers love Spider-Man, much more effectively than by cheering and clapping while he punches the bad guy. (I would love, though, to see a throwaway scene in a future Spider-Man movie in which New Yorkers pointedly look away when he is unmasked in public, or witnesses to an unmasking give wildly false descriptions of him.)

After the final fight with Doctor Octopus, the removal of the mask allows Peter Parker to appeal to Otto Octavius. Spider-Man has defeated Doctor Octopus (who is controlled or influenced by the AI of the mechanical arms), but Spider-Man will never convince Doc Ock to shut down the experiment. And Spider-Man has no foundation to appeal to Otto Octavius. Peter Parker, however, has an existing (if brief) friendship with Otto Octavius — they connected intellectually and personally before the accident ever occurred — so Peter can appeal to Otto to remind him of the good and ethical man he really is. The mask has to come off for him to talk man to man, instead of hero to villain.

Yes, the frequent removal of the mask can be awkward and contrived, but it does serve a practical purpose by allowing the actor to tell elements of the story through facial expressions rather than through exposition (and a character in a mask shouting “ouch” or saying “you’ve really made me angry” really is just exposition). Handled well, it adds emotional content and plot elements to the story.