Why does what Meg sees make her angry? Why does Meg thank Whatsit, Who, Which's friend for making her angry?
Meg becomes angry when the Medium shows her a vision of her mother at home, writing a letter to her absent husband, sinking into a depth of unhappiness that she does not allow her children to see. This gives Meg a larger perspective on the power of evil, not just that it has captured her father. She sees that, an attack on one person affects all those who love that person, and thus continues on, spreading like a cancer.
Meg's anger is righteous anger, directed at evil and the consequences of evil. This spurs her to action, willingly going to Camazotz to free her father. This anger allows her to push past her fear and her self-doubt. Her focus ceases to be on herself but on her father.
Mrs Whatsit's parting words to Meg are, "Stay angry, Meg. You will need all your anger." In stating this, Mrs. Whatsit ascertains the nature of Meg's anger and encourages her to retain it as a means to focus her energy, especially as she encounters the nature of evil itself.