If you tell me you're going to make my life easier by teaching me "Roman numeral Analysis", I'm gonna run away. That sounds scary and vaguely reminds me of Latin class.

"Nashville number system" sounds easy to master. It's country, and country has a well-known self-imposed reputation as simple. (In truth, country can be just as complicated as anything else. But I'm talking about first impressions.)

I used to be a part of a congregation whose band spoke in 5ths and 7ths and I had no idea what they were going on about. And then I learned that part of joining the band was learning the Nashville system. It's just the simplest way to get everyone on the same page, and when you say "Nashville" musicians immediately relate to what you're saying.

A reason for the usefulness was how recorded music was made. The recording musicians had to be able to choose a key that accommodated the singer's range, on the spot. So a transpose-able format was ideal.

I think the industry in New York had a different scheme, which was to write for a "standard" male tenor voice, and rely on the musicians to handle exceptions.