DEFCON informs what posture the US Military (and thus, any other institution globally that cares what the US Military is doing) is taking. It has very tangible effects on US Military personnel and how quickly they are expected to respond, and the storage state of various assets. For example, at DEFCON 3, you would expect the USAF to essentially have weapons staged at all times to be fitted to aircraft; maintenance crews ready around the clock, and so on.