I’m not a bar person, but I regularly visit a nearby bar on Tuesday evenings. It is because that night is the jam night. Each musician gets to choose two songs to play or to sing. Other musicians will go along with these choices and stage a performance. The sounds of keyboard, guitars, bass, drums, vocals, and sometimes a saxophone, are fused together and exemplified by loud speakers.
Although I am always a spectator, I find the whole experience fascinating.