I played a gig last night that turned into a practice session. It turned that way because nobody was there. Why did we let it get that way?

Playing a gig is the act of doing work for others and getting paid for that work. It's not for practice. It's for connection with someone else. It's also for bringing that someone else into an experience.  

Who goes to a show to watch a band practice? Unless you want the experience of watching artists practice, nobody.  

You go to a show because you want to feel part of something bigger than yourself.  

This post is not a critique of the bandleader. I admit that I had a hard time treating the work seriously. And I acknowledge that a prior version of me would have given the current version a hard time for my lack of serious attention.  

In any work you do, remember what that work is for and who it is for. It is not for you to practice your craft. It is for others to experience something that matters to them.