There are the reasons why I paid:
1. The course is awesome and I want to support the authors.
2. The course I paid is new learning area I never use professionally during job but I wish to use at my job (or new job). Instead of say "I interested in XYZ", I said "I interested in XYZ so I took online course to learn" at my interview. This is to partially solve chicken and egg problem.
3. For serious courses, I pay so I can have pressure and motivation to complete it.