Mike is the reason they don't die to the mindflayr - he is the only one will confides in and understands he's a spy, and comes up with isolating him I'm pretty sure. Also I think Mike is essential to eleven sticking around, but idk about that one as much
Johnathan/Nancy was to get closure for barb and presumably to pay off the tension from the first season between them. Also gives a reason for Steve to be a monster hubting bad ass since he's now single and hangs out with 14 year olds
Billy makes no sense tho and should have banged the mom