Discipline is valuable because time is valuable. We know we have a limited amount of time.
We do not need to specify here what make a good use of time. For the sake of argument, let's grant that if we spend our time on something we think is valuable, then it is well spent. Even then, we have the possibility of spending it poorly.
In order to spend time well, we need not only to have the time itself (we are not obligated to spend the time according to someone else's priorities), but we must also have the energy to spend the time well.
When we have a time slot open, but we do not spend it the way we want to (we regret the way we spent the time), it points to a lack of something, but of what? The answer is energy. We may have managed our time well, but mis-managed our energy.
Discipline is the skill of managing energy. Without it, we will waste precious time because we don't have the energy necessary to use it the way we want to. That is why discipline is valuable.