Please, explain why students apply for audit classes.

to audit a course
 [v. phr.] to attend a course without taking it for credit
Notes: If a student wants to officially register for a class without earning a grade or credit, the course may be scheduled as an audit. Regular tuition fees may apply as if the course were taken for credit but the course does not count towards any degree, certificate, or diploma. A student auditing a course may be required to participate fully in the class. Expectations should be clarified with the course instructor at the start of the semester but students usually do not submit work or sit formal examinations.
Example: You should process a registration drop/add form to schedule an audit.

What for an audit class if you don’t get a credit for it.

I audited several university classes. It is a matter of personal, academic or professional interest.