Without knowing exactly which play you are referring to, it is difficult to give specifics, but Shakespeare's servants often have a series of crude lines. One reason might be comic relief. Many of Shakespeare's play are tragedies. The crude servants offer the audience a much needed break from the drama and offer a chance to laugh. The servants are often able to set the scene for us. For instance, the servants at the beginning of Romeo and Juliet draw in the audience with a very crude discussion. They also let us know that we can expect a different kind of love story. The Bard has already shared that we will meet a pair of star crossed lovers, but the servants show us that there will also be lust and attraction between the young couple. The crude jokes and suggestions help to make the characters a little more three dimensional for the audience. While the characters proclaim their love and the world steps between them, the servants offer another vantage point and confirm what much of the audience is already thinking.