I've got a rather large collection of large photo images. Images which I can and do want to work on, crop, tune brightness, contrast, etc, before publishing them. Webapps right now are amazingly primitive and crude for even the most basic of workflows, especially when you're working with raw images.
Additionally, in order to save the images I want how I created them, I'd have to pay a non-insubstantial amount of money to store them. By saving everything locally, I can have the full-resolution image for my own usage, and just pay the occasional cost to upgrade/replace hard drives.
Furthermore, I'm almost certainly going to have to have a copy of many of the images on my computer anyways. A lot of the images I make, I tend to want to share in 2 or more other places, not just a random web album. So again, the webapp usage story falls apart.
I'm also not always in a place where I have internet access which is particularly fast and reliable. So, needing a webapp in such a situation means that I can't edit and arrange my photos at all until I get to a location that has internet access. Not everyone wants to be hyperconnected all the time.
Webapps mean you're even more beholden to someone else for functionality than a desktop app. You are limited in how you can post your content, where you can post your content, and even what content you're allowed to post. A desktop application has none of these limitations, whereas they're inherent flaws in the webapp ecosystem.