Where and why do Calvin, Charles Wallace, and Meg go with Mrs. Whatsit, Mrs. Who, and Mrs. Which in the book A Wrinkle in Time?

Mrs. Whatsit, Mrs. Who and Mrs. Which first take the three young people to the planet Uriel, described as the third planet of the star Malak. They go there "to catch our breaths," as Mrs. Whatsit puts it, because it's a safe planet, but also so that the children can be shown the "Dark Thing," evil itself, that partially shadows some planets and has completely taken over other planets. The six companions then travel to...

Mrs. Whatsit, Mrs. Who and Mrs. Which first take the three young people to the planet Uriel, described as the third planet of the star Malak. They go there "to catch our breaths," as Mrs. Whatsit puts it, because it's a safe planet, but also so that the children can be shown the "Dark Thing," evil itself, that partially shadows some planets and has completely taken over other planets. The six companions then travel to a planet in Orion's belt, so that Calvin, Meg and Charles Wallace can meet the Happy Medium and see the earth in her crystal ball. They see that the earth is partially covered in darkness and yet still has goodness and light in it, brought by religious figures such as Jesus and St. Francis, and by artists and visionaries who keep hope alive. They also see that darkness can be eradicated on a planet:

Suddenly there was a great burst of light through the Darkness. The light spread out and where it touched the Darkness the Darkness disappeared. 

This happens often, the children are assured. All of this prepares them for their journey to Camazotz, a planet completely taken over by evil, to save their father.