To indicate, very briefly, that someone's read a post, and wants to let the poster know that they're there and they've read it, but hasn't got anything to say about it that they can articulate in words particularly easily, people leave comments that have little to no actual content.
Some common forms of this are the single period, the elipsis, the html comment, and the html non-breaking space.
HTML comments: <!-- comment goes here -->
HTML non-breaking space:
Note that the HTML escape characters for the nbsp sometimes do not work when they are in caps.
I, like elynne, may start doing this to signify my presence, though I do generally leave an assload of comments anyway.