As an outside observer, it seems that providing the desktop app allows the developer to have a freemium model without incurring additional hosting costs for those users. Users who want cloud storage offset his costs directly with the $5 charge.
Just guessing
Side Note, for those who haven't downloaded the app. There doesn't seem to be a mention for the cloud storage option on the home page. The app stores data locally, but offers the option of cloud storage with additional benefits for $5/month. Screenshot: http://i.imgur.com/zWYpv7I.png
Not being a webapp is a huge plus for me if I'm going to use a piece of software for something important (when the use isn't inherently tied to the web).
The people who make this could sell to GoogleAppleSoft, get hit by a bus, or start charging stupid money for it, and I'll still have a copy and all my data. A webapp by a company I've never heard of before needs to do much more to reassure me that won't happen.
If you were to use this, would you really want to give all your detailed financial information to someone else? Who else do you give that info to? So desktop definitely good idea.
I'm curious as to why have you decided to package this as a desktop software, rather than a web app?