A little history to clear up some misunderstandings/misinformation about the numbers system. It was developed in Nashville in the late 50s/early 60s (can't recall exactly when), and it is most often credited to Nashville session musician Charlie McCoy. Charlie saw vocalists using a similar method and thought it would speed up the recording process for studio musicians.

That was the primary reason it was developed and implemented. Not for musicians who didn't know how to read music (although there has been a great number of amazing session players over the years who can't read music at all), but to speed the process up. In those days, and in most of my years in Nashville, the artist didn't write their own songs for the most part. Time was also of the essence, so producers were trying to get as many songs recorded as they could in the allotted time.

Typically, a song is brought to the studio, in those days maybe on a reel to reel tape, acetate, or even just a guitar & lyric sheet. The musicians learn the song on the fly. They don't hear it beforehand, rehearse it, or have any prior knowledge of the song until they hear it in the studio. The musicians usually write the chart themselves, as they are listening to it. Some people write charts beforehand (I always do) but others don't. Let's say it's my song, and I wrote it in the key of Eb. But the singer needs to sing it in G. Imagine all the work and time it would take to transpose a chart written in the standard Eb/Ab/Bb method. With the numbers system, the chart works in any key. So it's a huge time saver. I've had many, many recording sessions where we've tracked 6 or more songs in 3 hours, in large part because of all the time saved by using the numbers system on the charts. One day in particular I remember - I had a double session booked (that is, a 10am and a 2pm, 3 hours each). I wanted to get 12 songs done, so we started an hour early in the morning (9am), and as I always do, I had a couple of extra songs ready in case one or two just didn't come together. So the session was from 9-1, then a lunch break to be followed by another 3 hours from 2-5. At around 12:40, my session leader/piano player said "Well guys, should we go ahead and track the 10th song now or go to lunch early?" We wound up going ahead & cutting it, so we recorded 10 songs in 4 hours. Here's one of them, if you're interested in listening... https://www.reverbnation.com/rogerbrown/song/3677243-it-lingers

Also, it works perfectly in any genre of music, including jazz - although the charts are a bit more cluttered. It's interesting to me that when I came to Nashville, I wrote charts in the standard format. Now I don't know that I can. If someone asks me what chord I'm playing, I say "it's a 3b" or "it's 7b+9". On occasion, someone will say "no, what's the name of the chord" and I have to stop and think about it, because I'm so used to thinking in numbers now.

Like Charlie, I'm not trying to convert anyone to the numbers system. Use what you're comfortable with, there's no right or wrong here. Just pointing out that it's an extremely useful and effective way of writing charts, especially if time is a consideration.

http://www.rogerbrownsongs.com
http://www.nashvillesongwriters.com/roger-brown