If Hyperlapse is so cool, it makes you wonder why it’s built as a standalone app, rather than a new feature of Instagram. (...). So they split it off into its own product. “We didn’t want to create a special use that would just be hidden,” says Mike Krieger, Instagram’s co-founder and CTO. - Wired
This, opposite to what Facebook or Foursquare are doing, is a good reason to make a separate app.