What is the purpose of the exchange between the servants? What purpose does this scene serve to the play overall?

The scene you are talking about is pretty minor. I'm not sure there is too much to read into it. Peter and the other serving men are complaining about work not being done. They talk about guests with bad manners, scraping plates and basically preparing for Capulet's big party. The scene serves to set the stage for the festivities. It shows much pop and circumstance. It also, indirectly, sets us up for the big meeting between the star-crossed lovers.