I think most of us understand that our mobile devices can track the you-know-what out of us; even so, it's a little creepy/weird to actually see the information that is being gathered. Location data is "helpful" in the sense that the information is mostly used to provide features like drive time to and from work, including traffic warnings.
Even so, that information is being gathered and kept if you use default settings and make use of things like Google Now, and the various mapping programs.
For Android users, if you are logged in with your Google account, you can put this URL into the address bar to see your location history:
For iPhone users, go to Settings > Privacy > Location Services > System Services > Frequent Locations to see where you have been.