In the olden days (you know, ~2008) it frequently was the case that if you didn’t read something as it was sitting there in front of you, you never would, and you likely forfeited many things you wanted to read because you just didn’t have a source-agnostic, friction-free way to store them for later. Yes, you could have just opened your 117th tab, or added it to your highly-structured bookmarks collection, but we all know that’s no way to live.
Instapaper is one of those amazing little services that I take for granted. I use it almost everyday and have stored thousands of web articles on it over the years. Honestly, I don’t know how I ever got by without it.