... I find it surprising the number of musicians who don't understand or haven't mastered the number system.
It is one of the single, most important elements of playing by ear.
On a grander level, it allows musicians to speak on a "universal level." We can describe what we're doing in a song without relying solely on letters. It's not, "D minor to G dominant 7 to C major 7" anymore. Now it's, "2-5-1" in C!