It helps with sound damping. A room without it needs a lot of soft surfaces to prevent it becoming an echo chamber. You can hear rooms “boom” after this is done.

Because when people remove it, they quickly learn why it was there. This is called acoustic ceiling covering. It was fine until 2000s home renovation shows started shitting on it because the point was to change and spend. Once removed, the rooms become echo chambers. While very early examples were made of asbestos as fire retardant, they were mostly made of vermiculite in the 70s and polystyrene after that. They were used by architects in the transition era post 60s where houses started to get built with prefab steel reinforced roof trellises which allowed much larger rooms and open concept designs. But big rooms get echo unless you add soft surfaces.

Builders in the US loved this covering because it was the cheapest way to finish a ceiling with least labor costs. One guy with a sprayer could do a whole house in a day.

It is a quick and easy way to hide bad drywall work. The texture makes imperfections blend in.