All fast running is not the same. There are a few speeds exceeding the natural jogging pace that competitive runners routinely hit in their training. It's good to hit them all because each contributes to fitness development in a slightly different way than others.
Even the most highly competitive runners jog most of the time. Easy running is great because the more of it you do, the fitter you get, and because it’s not terribly taxing you can do a lot of it. Faster running is more taxing, so it can only be done in small amounts. But a little goes a long way, especially when faster running is layered on top of a high volume of easy running.