One of us should write up a better description at some point, but I
think the main value from a science perspective is that:
(a) it gives outsiders and newcomers an idea of which ideas lots of
others currently find exciting, and
(b) the commenting features allow for real-time public discussion of
papers. these conversations used to happen behind closed doors, but
now anyone can see them, and authors often feel compelled to respond
when people point out bugs in their papers. Also, they move
scientific publishing a bit closer to the wiki ideal.
Another way to discuss it is as being like stackoverflow and related
sites, but with the discussions being anchored by arxiv papers instead
of by questions.
aram