Why were masks worn in religious ceremonies?

Masks were worn for a couple of reasons. Firstly, if the wearer was trying to invoke a certain god, spirit or animal, wearing the mask of who they were invoking helped them to do so. Secondly, masks added an anonymity to ceremonies that helped observers believe the ceremony better - it was easier to see an anonymous person doing something and to get something out of it than it was to see your neighbor Joe who lives next door doing the same thing. It made the experience more special.