Tracking is a term that includes many different methods that websites, advertisers and others use to learn about your web browsing behavior. This includes information about what sites you visit, things you like, dislike and purchase. They often use this information to show ads, products or services specifically targeted to you.
Maybe you are asking: What we need the User Agent for iPhone? In fact you don’t need it a lot if you have an iPhone, if not I’m sure that you need. For example there are a lot of service that you can access ONLY from an iPhone. Another good point that I started writing this post is reading for free around 20 different magazines (for free and legally).