Or so I’ve heard.
maybe some might. though this seems to have become a cliche.
i think it is focusing your self on something besides daydreaming like an object or your breath or part of your body (there is belief in energy spots in the body).
The navel area is the site of the manipura chakra in Hindu, and the dantian or hara (lit. “belly”) in Chinese and Japanese thought, which is considered both the physical center of gravity and the location of one’s internal energy, or chi/qi/ki. That said, I always thought “contemplating one’s navel” was just a humorous western saying meaning to zone out on something of little consequence.
Same reason early '90s indie rock bands gaze at their shoes.
I thought the same and always thought it had a derogatory connotation too. A way of saying that what they were doing was useless.