Think of IB as a complete package. It is curriculum and course work taught to universal IB standards. The International Baccalaureate Organisation authorizes schools to offer its programmes. (You don't just hang out a sign saying you are an IB school!) The teachers have to undergo a thorough preparation for teaching the IB programmes. The coursework culminates in examinations set and scored by the IB organization.
IB and AP are all about excellence and high standards. Any prep school which offers IB or AP is making a statement. They are investing enormous amounts of effort and resources into achieving the best possible standards of teaching for their students. The result hoped for is a class of graduates who are ready for university level studies.