“It showed all the “processing” animations for a couple of seconds. While I was waiting for the SVGs, it opened a new page asking me to sign up to download the generated SVG files.”
The PNG-to-SVG site already had his files on its server before it mentioned an account. That order is deliberate. Time already spent is the leverage, and the fake processing animation keeps you invested. Sign-up counts go up and the growth team calls it conversion.