These jailers might only appear in one scene, but they provide some much-needed comic relief. Their discussion with Posthumus about whether he is ready for death steals the scene. Just like the Porter in Macbethor the gravediggers in Hamlet, these lower-class characters leave us laughing about a not-so-funny topic—death.
Shakespeare loves to give us a lot of dark stuff about the meaning of life and death, and then follow up with a comic interlude. Why?
Well, for one thing, it's funny. No, seriously: we need a break from all the tension and death in this play. The jailers are Shakespeare's way of giving us a bunch of wisecracks to have fun with. These punsters take the play's serious themes and give us a humorous counterpoint; this helps us see that in real life, the serious and the not-so-serious are always intertwined, as if nothing is ever purely tragic or purely comic.
These two punsters also set the tone for the final scene of the play. The humor in their scene lightens the mood just enough to make all the happily-ever-after at the play's end seem like it's not just coming out of nowhere.