[dspillett]

In a running gait, a heal strike usually means you are hitting the ground with a straighter leg so more of the energy is going through joins instead of being absorbed by muscles and other squishier bits. This can lead to excess wear to your knees and other joints which over time (especially if you don't give yourself enough recovery and healing time after hard training periods) can lead to permanent problems.

Also, as well as the shock to your body of absorbing the impact energy this way, it is less efficient: if your more springy bits absorb the energy instead of your solid structure, then they can release some of it back in a useful direction as you push off again with that foot making it easier for you to maintain the same pace. On top of that, because the “stop” of each stride is less abrupt less energy is lost to entropy through your flesh jiggling because of inertia.

I'm pretty sure this is all different for a walking pace though, which would explain you seeing a benefit from the opposite advice. With a walking gait your forward momentum is provided & maintained by the foot rolling over the floor and pushing sideways against it, rather than striking and pushing away from it like in a sprint, with running being somewhere between (running and springing are different as much as running and walking are: a sprint is a period of acceleration or maintaining top speed, a running posture is for efficiently maintaining a cruising speed or more slowly accelerating - the mechanics, while similar, have key differences).