Crazy US shit, never heard of it anywhere else. Basically a group of your neighbours who say what you can and cant do to your own property.
Plus if the HOA members don't like you they can make your life difficult. Planning permission affects everyone equally. If a neighbour can extend a property then more than likely you can too, if you are refused it is likely all similar applications would be refused, you don't know the planning officer and they don't know you so no bias can be involved which make the whole process much fairer.