Why must Meg go alone to Camazotz? How is her relationship with Charles Wallace important to her ability to free him?

Meg must go alone for two reasons. First, she has been depending on others, such as the three "witches," and they see it as time for her to act on her own, and to gain maturity. At the same time, they want her to see how connected she is to her family. She acts as if she is isolated and weak; they want to show her she is strong and connected, and that her connections— and specifically her love for her brother—are very real sources of strength.