How is "set -g" really different from "set" on fish shell?

I am trying to understand how the set command works on fish shell . For example, to remove the greeting message, the fish manual suggests to: > To empty the text, with the default greeting function: > > set -U fish_greeting > > or set -g fish_greeting in config.fish. But why it is suggested to use set -g fish_greeting in config.fish instead of just set fish_greeting? What is the practical purpose of -g here?