Being a musician I have realized that the number system is incredibly important. I am shocked and suprised at the number of musicians who haven't mastered this amazing concept yet. Keep in mind that this only applies to people who play scale instruments such as bass, guitar, piano/keyboard.
To put this universally it's an important element to playing by ear. Secondly, this allows a musician to communicate without speaking in letters.
For example,
In the key of C going from a D minor chord to G major chord then a C major chord. Do you see how difficult that is to say. It's just a lot better to just say 2-5-1 in the key of C.
The best part of this is that it can be "translated" in any key. The only thing is that you just need to remember the different tones in every key.
In Bb major, it’s:
C minor 7
F dominant 7
Bb major 7
That's it. It's really a fun and simple concept.
When you number the notes in the scale. It's an easy way to put it.
For example, the C major scale is: C D E F G A B C.
I just number this scale starting at 1.
C is 1
D is 2
E is 3
F is 4
G is 5
A is 6
B is 7.
The number system is just a fun and unique concept. If you are new to this don't be afraid and give it a try.