The groundlings were the uneducated commoners who attended the plays. They were called groundlings because they paid the lowest amount of money to enter the theater, and they had to stand on the ground next to the raised platform stage. The groundlings often went to the theater to see clowns and fools and other comic characters. They wanted to laugh at such characters, and most playwrights included such foolish characters to satisfy this part of their audience.
In Much Ado about Nothing, Dogberry is this type of character. In one of his earliest comedies, Love's Labour's Lost, Shakespeare created a comic constable named Dull. But Dull was not nearly as well developed or as humorous as Dogberry is. Dogberry is a delightfully comic character that immediately grabs the audience's attention and holds that attention for as long as he is on the stage. Shakespeare delighted in playing with language and subverting language. With the character of Dogberry, Shakespeare found an ideal figure to explore the limits of language play.
Dogberry is a caricature or comic portrait of the night watchmen that so often could be found in Elizabethan villages. Apparently, such watchmen or constables were not very effective. They were neither smart nor competent. Moreover, they were afraid of criminals and avoided them as much as possible. Although they were supposed to keep the peace during the night, they much more frequently spent the entire night sleeping on a bench in some churchyard or other quiet place. Dogberry is funny because of his problems with English, his linguistic muddles. But he would also be funny to Renaissance audiences because many of them had known such constables in real life.
One of the great elements of this comedy, though, is that Dogberry is also crucial to the plot. Without Dogberry, the play would not have been a comedy. Dogberry exposes Don John's treacherous actions and thus sets up the play for the happy ending. Moreover, the scenes with Dogberry provide comic relief at such times in the play when the vicious scheme of Don John appears as if it is going to be successful. Dogberry thus also helps to create a lighter, carnivalesque atmosphere. The audience does not fear a tragic ending for the play because Dogberry keeps them laughing.