There's something enormously infuriating about having a WiFi connection drop in and out as you're trying to work in the city. Sometimes you don't want to have to sit in a sweaty internet café in order to get online.
And there are definitely people out there who won't want to visit the Apple store (usually for fear they might buy something).
Mobile broadband is an alternative to WiFi and home broadband, because it uses mobile phone masts instead of phone lines to connect.
It's pretty quick these days, with speeds over 3Mb quite possible in most areas. And the best bit is that it's not even that expensive - for some people it might even be the cheaper option.