[khedoros]

The image itself is customized; it's not a vanilla copy of Raspbian. So, no matter what, you'll end up downloading ~1GB of data (less, when he's got the Lite version built).

One benefit of this, over doing it online: you can download the file+program, disconnect from the net, and customize as many versions of the image as you'd like without extra bandwidth use. There are a lot of Pi users with limited internet access.

Another benefit: Doing it online implies at least a two step process. Go to the site and build a setup (with either an account system or an import/export feature to save progress and make it available for re-use), tell it to build the image, download it, and use a separate program to flash. Doing it through a native app eliminates the separate program and the question of whether or not you need to create an account on their service. Plus, when their site goes down eventually, I've still got a working, independent program. When they introduce a new version, I've got the option to stick with what I've got or upgrade (if I judge it to actually be an upgrade).

Web apps have their place. This thing is one that makes more sense as a local app to me.