Why are the Marthas important in the story Speak?

Not really. The Marthas are a group of preppy girls who get good grades, suck up to teachers and do things to help. Heather wants to join their group. The Marthas seem to test her, and if she doesn't do well, she's not allowed to sit with them or do stuff with them. There are senior Marthas and new Marthas, and Heather is new. They haven't quite accepted her yet.