It's fear mongering time boys and girls. (Spoiler warning!)
Settting: the movie is shot through a hand-held camera for the entire duration of the movie. The precedent for this type of cinematography was already set by the Blair Witch Project. I believe the psychological effect on the audience is two-fold:
- To establish a stronger sense of realism and authenticity.
- By capturing the surroundings through a confined tunnel-like vision window, the viewer is left guessing as to what is going on in the periphery.
Although the big bad über-creature obviously cannot reach into and roam around in the narrow subway tunnels, it gets help by scary-looking hostile spider-like creatures, the size of dwarfs. I believe that these smaller critters, which have no semblance to the bigger über-critter, were put into the movie-script to imbue the notion that even the underground in general, and the subway in particular, offers no safe refuge to whatever danger is brought to bear and wreaking havoc above the ground. Of course it just so happens that one of the main characters gets bitten by one of the smaller critters and later on dies a violent death, thus further adding to the viewer's anxiety towards reception of the smaller critters.
In other words, during a calamitous event, whether you are above the ground or underground: be afraid.... be very afraid.
It is to be expected that the only governmental entity possibly capable of giving sanctuary to the victims of the calamity is the armed forces or so the movie suggests. This gives the viewer the impression to place trust, preferably unconditional, in the military during times of social upheaval and mayhem.
Although the remaining characters of the movie get away in a military helicopter the main critter manages to thwart their escape to presumed safety by swooping the chopper down to the ground, thus setting in motion the chain of events that leads to the demise of all remaining movie characters. Even the army does not seem to be able to guarantee safety during times of extreme calamity, although the movie suggests it is the only option that is extended to you in regards to survivability.
It is not a terribly big stretch to think that the creatures portrayed in the movie are not that particularly realistic but that's not really relevant to the programming imbued, I believe. The function of the movie seems to be to instill more fear into the public by promoting the notion that safe havens are found neither above the ground or under it. Another function is to get the public used to a police state type of society and to place trust in the military above all else although even they cannot warrant public safety under some circumstances.