5.6Why Roman numerals are so useful

Suppose someone says:

“The progression is I–IV–V–I.”

You can play it in any key.

C:

C F G C

G:

G C D G

D:

D G A D

B♭:

B♭ E♭ F B♭

Roman numerals preserve harmonic function while the absolute note names change.

That is why musicians talk this way.