You drag their "Read Later" button to your toolbar so that whenever you're at a site, you just hit it and it's saved. Later, you go the very minimal Instapaper site, and see what you've accumulated.
This is great if you're following a topic such as the 700 MHz auction and you're scanning a bunch of sites, and eventually want to use material/links in a blog post.
David thinks that others should be able to see what you've collected in the spirit of the folksonomy movement typified by such sites as Del.icio.us, Digg, Tumblr, etc. I don't agree. If you can't synthesize the information you're reading, you're not providing much of a service to your readers.