Much like Android apps that offers a choice between ads and paying to make them go away. Opera was initially for pay only, much like Netscape Navigator. IE was the really odd one out by being offered for free (and why Netscape pulled Microsoft to court over antitrust charges).

Opera introduced the banner after Mozilla was formed and started offering first a rewritten Netscape package (these days called Seamonkey) and then Firefox (after some back and forth about naming).