Junior colleges cost less per semester and per hour than four-year colleges, public or private. Over the course of four semesters, you can save tens of thousands of dollars before transferring to a four-year college.
Some young students aren't ready to leave home yet, or they prefer to set a solid academic foundation before traveling to another city. Students with families also prefer the local commute to the junior college instead of moving, and everyone saves on room and board.
The average class size of most junior college is less than 25 students, whereas many four-year colleges have class sizes that range into the hundreds. Having a closer working relationship with your professor can improve your understanding and your grades.
Most junior colleges really work to involve the local community, often consulting with local employers to determine what skills and knowledge are needed in the local economy. This relationship can give students from the junior college an edge in finding a job.
Confirm before you enroll, but most junior colleges have fully accredited academic programs which easily transfer to the big schools. You won't need to worry about whether a particular English or math class will be accepted--it's already been cleared.
It's easy to get lost at a university with 30,000 students; attending a local junior college with only a few thousand students encourages you to get involved in extracurricular groups with other students you see walking across campus every day. You'll often see many of the students you knew at your own high school, too.