National Council on Teacher Quality:

Though few would dispute its value, the job of providing apprenticeships for some 200,000 teacher candidates each year in real classrooms is a massive and complex undertaking. About 1,400 higher education institutions work with many thousands of school districts across the United States to place, mentor and supervise teacher candidates in what is popularly known as “student teaching.”1
Even as the profession pushes for more and earlier field work opportunities, student teaching is the final clinical experience.2 During the typical semester-long experience, student teaching candidates must synthesize everything they have learned about planning instruction: collecting or developing instructional materials, teaching lessons, guiding small group activities, and establishing and maintaining order–not to mention meetings with faculty and parents and, in some districts still, taking on lunchroom and playground duties. Passing (or failing) student teaching determines whether an individual will be recommended for certification as a licensed teacher.
Because few dispute the tremendous potential value of student teaching, even alternate pathways to profession, often criticized for taking too many shortcuts, generally try to provide their teaching candidates with some kind of student teaching experience, however abbreviated. Surveys of new teachers suggest that student teaching is the most important part of their teaching training experience.3

Related: Teacher colleges balk at being rated Wisconsin schools say quality survey from national nonprofit and magazine won’t be fair.
National Council on Teacher Quality website.