Simple question about drinking in clubs

Hello I have a simple question, I never understood, how come in some Kdramas when certain groups of, namely business men(or groups of guy friends), go drinking they get those private rooms in clubs and seemingly random girls are invited to drink with them, is that some kind of job? Are they just entertainers? Are they just there to pour drinks? Sorry, I'm not really sure where else to ask this I was just curious what that was all about.