Tragic Renaissance dramas are commonly tinted with humor based upon the fact that the playwrights believed the audience needed a rescue from the morbid and depressing nature of the play.
For example, in Shakespeare's tragic play Macbeth, the part of the Porter is included in order to allow for comic relief. The Porter, drunk from the night's festivities, inadvertently compares Macbeth's castle and Macbeth to both hell and the Devil. The Porter also rants about lust-fullness and what drinking does to one's ability to be able to have sex. After the scene with the Porter, the plays action returns to its intensely dramatic mood. The Porter can certainly be seen as a "mad" character based upon his dialogue.
Therefore, comic relief is simply used to interrupt the tragic nature of the play in order to allow the audience, or reader, an escape from the intensity of the play overall.