ThisTo answer the OP:
1. People to play golf with
2. Access to the course whenever you want, e.g. just to play a few holes in the evening
3. Playing in competitions and maintaining a handicap
4. Discounted guest fees to invite your mates
5. Access to premium tee times i.e. weekend mornings
6. Practice facilities (depends on the course)
7. Team golf
8. Convenience of storing gear at the club (e.g. locker and trolley store)
9. Discounted food and drink
10. For a private club: Some influence in how the club is run / satisfaction that it is being run in your interests
11. Access to quality: the better courses usually restrict visitor access or are prohibitively expensive unless you are a member