u/HeidiDover

The writers needed to make Seth a temporary character to make way for Sara Linton, an ER physician who eventually becomes Will's wife. On the show, Angie is a sympathetic character. In the books, she's pretty awful and makes Sara's life miserable. I think the writers needed to make Angie's life fall apart so she will relapse, lash out, and become a foil to Sara and a potential antagonist. I hope I am wrong because I love show Angie.

Also, the writers had no good reason to take out Amanda. That was a shitty, cheap, and lazy thing to do to drive the plot.