Auditing a class can be a way of seeing if you're interested in the subject matter. If you enter college with an unclear view of what you want to do with your life, auditing classes in several departments might help you discover where you belong. If your workload becomes so heavy that it's impossible to keep moving forward with your education, auditing can answer your questions without causing you undue stress over assignments or tests.
You can audit a class to learn the material but without the worry of what the grade will do to your grade point average. If you don't need the course to round out your credits but feel the information is beneficial to you, auditing allows you to learn and participate in the interactive portion of the classroom. With a particularly heavy credit load, auditing allows you the opportunity to learn what you like without increasing your work as you choose what you participate in and what you don't. You still walk away with what you learned in the class, which is the ultimate goal of all coursework.
Auditing can be a good way to determine your capabilities when it comes to course load. If you are returning to school or just beginning college, you might not know how many credit hours you can handle. By taking a lower number but adding an additional class via auditing, you can test yourself to see if you are capable of the workload. You might find that you can handle more than the average student or that you are better off taking fewer credit hours to keep your work up to par.
Some colleges allow non-students to audit courses. This lets you sample colleges, instructors and specialized subject matter. Sometimes retired people do this, sometimes high school students. You can find this entertaining or useful in making decisions. Especially if you are considering a specialized college such as art or medicine, this exposes you to the course to gauge your interest and ability. For high school students, auditing can help them decide between schools.