For some people it is an easier, more gradual transition, and a cheaper way to determine what you want to major in. It’s also intended more for kids who aren’t sure if they can make it 2 years, let alone 4. Instead of dropping out halfway through, they get a degree and then decide whether to move on.
It can also be a way to get college admissions for (often homeschooled) gifted kids who needed college level courses in high school but didn’t want to leave home at 14 or 15. I don’t quite understand the issues involved, but apparently some colleges (public ones???) won’t admit you as a freshman if you already have a degree. Or maybe the issue was getting scholarships. I’m still not clear entirely on the issues.