I think it helps them take a serious look at their lives. Habits, plans, emotional wellbeing. It's easy to get stuck in the mundane hum of modern life and rushing between appointments. Waking up early is their way of controlling that rhythm without giving up on any commitments and allows more time for them to reflect.

Now, as many successful people have introduced these habits and various books about morning people being successful have come out, it is associated with a very particular white collar executive identity that young ambitious people aspire towards. Even as social norms have changed and people may value norms other than corporate status seeking, the habit is still associated with success, self-discipline, and productivity. People who would like to assume the identity are those who preach the word most arduously.