For example: You come on to my site, I want to know how you're using it, I don't want your personal details, I just want to see how you're interacting with the site I've made for you. Why do I want to know? Well it depends on the purpose of the site, but for the most part it is so that I can optimise and improve what my site offers to you and others.
But you've politely requested that I don't track you. For starters this should only ever be a polite request, not a forced rejection of any tracking scripts. I have a right to track how people use my site. You have a right to privacy, but that's got bugger all to do with you coming on to my site, once you've made that choice you are within my domain, under my roof, living by my rules. Until you leave of course.
Some sites may respect that request, but they're the kind of site who have no need to track behaviour anyhow, and are likely not tracking to begin with. kind of makes the request moot.
People get way too offended by analytics tracking when it's there for their benefit. The internet would be one ugly place if webmasters and designers had no clue how people were interacting with it. If you want to go back to the dark ages then feel free to try. But you won't benefit from the advances we've made or are yet to make because of large scale, anonymous tracking across the web.
I've no respect for Do Not Track. It is a silly, backwards, progress-endangering concept that should be burnt on a pyre.
Think of a scenario where a site is maliciously tracking you, where a forced browser level request could to not track be sent, and maybe we'll talk. But then again I'll probably just retort that any malicious tracking will have a way around such a forced request, and so it's pointless.
Do Not Track is snake oil for the conscientious objector.