Well, suppose you know the numbers as they apply to all 12 keys in music. If you sing a song in the Key of C, and a friend comes along who sings that song in the Key of E - you don't have to even think about what the chords are, because you are thinking of them as numbers. Since you know what chords those numbers equate to in both of those keys, you automatically know all of the chords instantly. Through the ages I'll remember, blue eyes crying in the rain. Through the ages I'll remember, blue eyes crying in the rain.
Some songs have chord changes within a measure.
You will never be sorry you did. If you play in a band, or jam with a group, try to get the other members to start using the numbers too, and communicate with numbers instead of letters. I realize it's entirely possible that this explanation might generate more questions. Hope this helps you. And - please notice that there is no copyright on this.