Someone recently asked me that question after I told them I was taking a class this semester. I never thought of auditing the class. If I chose to audit, I would not have had to take exams, etc. Why would I want to do that? For me, the reason to continue taking classes is to exercise my brain. Studying for exams, writing papers, and reading for the class are all ways that challenge my "aging" brain. I enjoy being in the classroom, learning about different subjects and interacting with my classmates and professor.
There has been a great deal written about "lifetime learning". One does not necessarily need to be in the classroom to be a lifetime learner. Learning happens every day.
Are you thinking about returning to school? Auditing a class is one idea. However, community colleges are cheaper than the four-year university and are always a great option for those "older" adults who want to get back into the classroom. One can also participate in personal and professional development courses that many universities offer. Don't be afraid to get your feet wet. There are many ways to ease back into the classroom. Google it and good luck! Stay tuned . . .