Why does "Do Not Disturb" have an option to allow calls from "Everyone"? What's the point?

I agree, the option to allow calls from "Everyone" actually kind of defeat the purpose of the "Do Not Disturb" feature. But please know that this feature does not only silence calls but it can also stop alerts, and notifications that you get while your device is locked. The said option is only intended when you absolutely need to let calls through.

No. With the option set to "Everyone", all calls will come through but no other notifications like from Messages or those sent from third-party apps or games.