'Granting posting authority' allows the account to do the things you give it permission to do in a reversible way.
If you just gave the apps your key it would be big headache to change keys if they misbehaved with your permissions.
So this function was created to make permissions revocable while granting access to use of the key.
If you use peakd, they have an interface to grant and revoke authorities.
ohhhhhh. omg great explanation. I spent (literally) hours, googling for this answer and it turned up nowhere. I will amend the guide to reflect, thank you again sir.