Apps exist for web based services the same reason those services like Facebook offer mobile versions. Web devs don't usually create a universally beautiful experience for every device, hence the need for an app. Sure, the experience is acceptable, but its not the same as having a perfectly formatted version.
I've really never noticed any watered down features that I missed dearly in many apps. Of course, there are a few aberrations, such as eBay, where you can't add tracking numbers for sales. But for quick, on the go access, I'd much rather have a properly formatted experience.
I really don't know why I just took the time to explain that because it should of been obvious, but, I digress.