A. Give kids some education (or educayshun, depending on the quality of the school).
B. Act as a sort of daycare, so that both parents can go to work and keep the wheels of the economy turning (or the computer mice clicking, nowadays).
C. Job security for the staff.
B+C can be easily done without accomplishing much of A, and the only thing than can guarantee at least some A is pressure from the parents. But in some places, concerned parents with enough time and energy to do that are so few that they cannot move the colossus. Moving away or finding private school for their kids is simpler.