The essay is like a sketch for a painting. It's brief and quickly crafted...well, relatively quickly crafted.
And it might suggest a larger piece of more extensive development, but has the merit of standing alone in an era of famously short attention spans.
Essays may be the 'trailers' for larger pieces, but at their best they are the kind of trailer that spoils all the best parts of the movie. The ones you see and say, 'I think I just saw the whole movie.'