I have been on the other end of this somewhat often, as one does when they take the subway to/from work every day.

Some of this is very self-serving for the median commuter as well. Someone struggling with a stroller or luggage is likely blocking some portion of the way for people getting in and out of the Subway, and the easiest way to fix that is just to help the person.

I've carried a number of strollers, suitcases and whatnot for people without barely a word. The same is true for helping people get their cars out of the snow, giving directions or anything else.