Because you don't want the trackers to know your real activity? It's pretty common now to do things like intentionally tagging the wrong friends in facebook be

Because you don't want the trackers to know your real activity? It's pretty common now to do things like intentionally tagging the wrong friends in facebook because people are paranoid that facebook knows too much. Even with non-techie people. They're creeped out. Throwing off retargeting is something a lot of people might want to do.