Because sometimes you want to change fields and your new field only requires an associate’s degree. Sure, if you had an associate’s degree already, you could spend 3+ years at a University to get a bachelor’s degree in dental hygiene, but you could also spend way less money and start practicing a year sooner if you did it as an associates at a CC.
People do this with double bachelor’s as well. It’s incredibly common with nursing where someone will get a bachelor’s and decide in their 20s or 30s to become a nurse. To do that, you go to one of the many “second bachelor’s” programs that only admit people who already hold a bachelor’s, and then give you a BSN in 2 years. MSN degrees are almost exclusively for BSN holders so switching to nursing and getting an advanced degree simultaneously isn’t really possible.