After playing in bands for a couple of years in high school, only then did I find out what a stage monitor was. All these years later, I'm still thankful for stage monitors, especially since two of the three Pull Tabs shows did not have them.
Until I played a show with a monitor, I just watched the guitarist and noticed where his left hand was and when he stepped on his distortion pedal. Since I could faintly hear his singing voice, I had to go off of body language. That's how many songs were written in a big, echo-y garage, so if I ever sounded like I was off that's why.
I don't forget that experience and never take monitors for granted. I've played shows where there were monitors, but I couldn't really hear anything. If I had to narrow down what was most important, the bass guitar and second microphone would not make the list. Let me just hear the lead singer and the rhythm guitar and I'm good.
There's another example of how important playing in bands in high school shaped me and many people I know. Now back to writing that book on the subject . . .