Q: Why must I use iTunes and not a webbrowser?

You mean for making purchases at iTunes Store? I haven't used it in a long time, but iTunes installs an ActiveX control called iTunesDetector Class that can be enabled in Internet Explorer. I also see it listed in Firefox add-ons, but I'm sure I have never used it with Firefox. I get the feeling you are a Windows user who would rather not install bloatware like iTunes, which I can certainly understand. It's really only indispensable for those with Apple mobile devices to support.