Which was inspired by the Readability experiment[1]. Not to mention that Safari has a similar feature now. Overall I like this approach better because I don't want to limit myself to a tiny subset of the web.
As far as I remember, the Instapaper simply saves web pages for reading later. Although it may somehow facilitate easy reading of long text content (e.g., by moving it into another reading platform like Kindle), but not too much.
[1] http://lab.arc90.com/experiments/readability/