As the lower text says, there are several possible targets besides the abdomen for front kicks, most notably the legs.
From my experience, using the whole foot is good for moving the opponent, may it be the leg to compromise balance and rhythm or the whole body (abdomen push kick). Sure, you could do that with the balls (abdomen, maybe even bladder) or the heels (leg) for more damage just as well. But for that, you must have a perfect target. And when you set up techniques by moving the leg or need to make some space by a push kick, your focus is elsewhere and thus it is perfectly valid to make sure you get enough thrust behind that kick and into the opponent. That's easier done when using the whole foot. Instead of the leg you can also aim for the outer grain regions. Goal is to turn the opponent and bring them off-balance, which opens them up for e.g. a hook punch as follow-up.
As you may have noticed, I mentioned to use the heel of the foot against the leg for damage. This can be done by frontal load with the heel into the muscle or by a "hook", where you basically do not do a front kick as such, but move your heel above the opponent's leg and then let your heel sink into the thigh from above. The goal here is to damage the muscle and impede footwork. Using the ball of the foot does not make much sense here as the danger of not perfectly hitting the mark and compromising your balance is much higher.
I've also heard people mentioning using the heel with a front kick towards the sternum. Doesn't do as much winding as hitting with the ball of the foot there, but allows for different mechanics which also uproot the opponent and make them topple backwards. Also, blocking arms are less of a problem. Only makes sense if aiming significantly above the hip line though. Personally, I've never used that as a teep kick into the bladder is easier and less risky.