As far as I understand (I'm not a runner), running should really be on your toes, and that's how professional and historical runners always used to run, though the invention of shoes and heels on shoes has made us walk and run more on our heels, which gives us bigger shocks on our knees and spine, which makes the cushioning necessary. My impression is that this is actually a wrong technique due to laziness, but I could easily be wrong; maybe cushioned heel-running is better than toe-running. I don't know.