Maybe a better question would be "what use-cases would cause one to develop towards one or the other" ?

Ring-out requires both caller and callee manually accept the call. It is basically the system call both parties and when both answer the call, it connect them to the call. Ring-out can be used to call from and too any party.

Call-out is dialing from your number programmatically instead of you dialing the number. Call-out is bound to a device so you need a device id and that device must be "online".