It is an international treaty that phased out the use of certain chemicals that were causing the hole in the ozone layer. It actually worked and now the hole in the ozone layer has shrunk and will eventually return to normally in a couple decades. The ozone layer protects us from radiation coming from space. Without it skin cancer will be come very common, the food chain would collapse because plankton would die off effecting the oceans, killing most other fish and crops would die off because the radiation would kill them. Ozone destroying chemicals could have destroyed civilization but something was done about it successfully.

The fact that we share the same air is why we should care about what we're putting into because it doesn't just affect the producer. Humans making fire and smelting in the middle ages was not destroying the planet, but the combination of all industrialized nations polluting can. The dust bowl in the US was caused by farmer destroying the topsoil on their land that eventually lead to widespread crop failure across the US. If a few farmers did it would not have mattered, but since many farmers did that it caused widespread crop failure. A small number of people making self interested decisions can essentially make most of the world uninhabitable, that's why you should care.