I used to teach certification courses and people attend for different reasons. A common reason is that the test covers the things that you should know to be successful in a given technology and the exercise of studying to know those subjects improves your skillset. Some people just love learning and want the sense of accomplishment from hitting a milestone. Some people came from the government contracting sector where the number of certifications reflected well on contract bids, so the company encouraged their people to get the certifications. Another reason is that in a competitive market, where two resumes, experience, personality fit are similar, the certification might be the deciding factor.