You bought the ad-free version, they had a small banner on top. And of course there were key generators and such, back in the days there wasn't any online key validation. Or you could kill the banner with a local proxy. Still, I actually wanted to support the development, just like I donate to good FOSS software now, or buy android apps to remove ads although I'm already killing them all with adaway on a rooted phone.

Sure, there were free browsers out there, but back then Opera was really way ahead of the bell curve.