Why does every "free" image converter add a watermark or upload your file?

The economics of "free image converter" sites are wild — they need you to upload, see ads, hit a watermark or quota wall, then upgrade. The Canvas API has been able to do this work locally since Chrome 5.

I built Devool-pro to test the thesis: every image/video tool a developer needs, in one tab, no upload, no signup, no watermark. 10 tools in so far. It runs entirely in your browser.

Genuinely curious — is there a category where you think browser-only WON'T work? Where a server-side approach is fundamentally needed?