The meaning of "audit" is very unclear, even if we restrict attention to the U.S. The other answers do attest to certain realities in certain situations, and to the variation. Perhaps to give an overview:
"Audit" at a minimum just means "hearing" the lectures. It is completely unclear whether or not you have to pay, whether or not you have to do homework, quizzes, or exams, whether or not you are allowed to do those tasks and get feedback, whether or not your official transcript will show that you "audited" the course(s), and so on.
The particular question of whether you "have to do any work" is funny. Indeed, in most audit situations, an auditor is not commanded to do anything at all. That is different from the question of whether a sensible person could see the point in engaging with the material. Truly, the model of "listening" which is almost completely passive has the virtue of very low overhead, and may be appropriate in some situation. But, if one wants more, and has the time and energy, the fact that one is not required (by external authorities) to do something does not mean that one cannot choose to do it.
For that matter, many universities' official rules (often, in the U.S., that people have to pay something to be "allowed" to attend a course, even if they make no further demands on the instructor) are not uniformly conformed-to by faculty. That is, the official description of the rules-of-the-game may not be correct at all "on the ground".
The one most likely common feature is that "auditing" a course will not provide you with any certification that you are competent or expert in the material. (We can wonder whether "good grades" ever did certify this...) Thus, if one views "education" as a process of obtaining certification, auditing is not directly purposeful. On the other hand, if one views "education" as a process of acquiring information, auditing is nearly as good as any approach, except for the possible lack of feedback from the instructor. (In fact, in many situations, the "exercises" and such are significantly make-work, exactly because the instructor is tasked with generating a steady stream of "work", so it's not clear that anyone should be terribly interested in "feedback" apart from anticipation of the eventual "grade" and/or success or failure of certification.)