From the Solana Cookbook, I can tell from the diagram that an Authority can sign transactions on behalf of the account whereas only an Owner can modify it. I can also tell that ownership can be transferred whereas authority remains fixed.
Elsewhere on the internet, I read that you can omit Authority entirely. So what exactly is the use of the idea of Authority? Accounts are always owned by programs that can write to them but in certain situations, they also tend to require signatures from certain authorities, why is that?
As a follow-up, who decides the Authority and Owner of an Account?