I really cleaned out Gmail a couple of months ago and all I had to do was to filter for attachments over a specific size. How much different is your app?
The main benefit of using Unattach over simply deleting the emails is that you can keep all the other parts of your emails in your Gmail server. These are often valuable, don't take much space and can continue to be searched. Unattach also provides various facilities to make finding and storing attachments in custom directory structures easy.