Actually, everyone doesn't get a supplemental application. A person can be rejected prior to the school sending out supplemental applications. Some schools have applicants fill out the secondary app. at the interview. Some also selectively send out supplemental apps.
If you want to be seriously considered as an applicant, you had better fill out the supplemental app. Alot of times, they use these in the interviews to ask questions. Alot of schools have application fees, they won't review your material until the check is cashed. Meaning you file will sit in a pile until they have your money. Is it a way to get more money out of you? Possibly. But processing your information etc. isn't exactly free on their part. The school incurs some cost in processing your file. If not anything, it takes time. Time is money. So why should they do it for free? But if you want a chance to be accepted, you better cough up the cash.
Also, your application is not considered complete until they have all required materials including the supplemental app. So again, they won't review your info until the checklist is complete.
HTH