Why browser instead of app
Most people use a roof visualizer once or twice while choosing a roof. Installing an app for that is a lot of friction for a few minutes of use, and app-store visualizers tend to be locked to one manufacturer's catalogue. A browser tool works on the phone in your hand right now and covers every material.
On a phone
Open the page in Safari or Chrome, tap the upload box, and choose a photo from your camera roll or take one on the spot. The render appears on the same screen with a slider so you can compare before and after. Save the picture to your photos or send the link to someone from the same screen.
On a laptop
Same page, bigger screen. Drag a photo in from a folder or from a listing. This is the better way to compare several colours side by side.
For roofers
The lack of an app matters most on a sales call. You can hand your phone to the homeowner or send them a link, and they see their house with the new roof without installing anything. Roofers get a dashboard, history and a link for every render.
