I started doing it as a kid in order to break my reliance on side sleeping and help general posture. These days I'm less strict but benefit from being able to find a comfortable position anywhere. I'd recommend not resting your head on a pillow but using firm cushioning under your neck; that will very slightly lift and relieve pressure from your head and keep head/neck/shoulders adequately separated. A straw cushion (common in Japan, maybe elsewhere) is nice but a tightly rolled hoodie is almost as good.
Sleeping on a hard surface to avoid side sleeping - noted. Sleeping on my back will lead to snoring which is not appreciated by those around me. I will try the pillow replacement suggestion, though.
What's bad about side sleeping? Just wondering.
Probably more health zeitgeist of the '90s than any real concern. Your bed, body type, and sleep style might make one more appealing than the other.
I was a restless sleeper though and had/have extreme difficulty getting to sleep often; learning to sleep on my back helps. Expressed another way, if my brain says I have to be on my side for comfort, and I have to achieve comfort to sleep, then if that comfort isn't achieved no sleep for me. Getting my brain to accept "and now we lie down for sleep" without thinking about comfort let me be actually comfortable in more positions/settings.