Charter schools are commonly founded as magnet schools, or as schools for at-risk kids, or those with special educational needs.
Critics of charter schools as magnets claim they syphon off the best students and leave the public schools worse off. The NEA, the largest union of teachers, supports Charter Schools, so long as they have "the same standards of accountability and access as other public schools."
Their success has been a mixed bag. Someone Charter schools do very well, and some are outright disasters.