Because it's an effort to make public education last 16 years instead of 12 or 8. If someone wants to extend their education to specialize in some higher level job, then they can work (and pay) for it. If we really need a higher ed job, there's grants and scholarships and paid internships to help support students.
But to answer your question with another question, how does having more stockbrokers, political consultants, and social workers help me in my daily life?