Why does Cherry want to testify for the Greasers?

Ultimately, Cherry does what she does for Johnny, Ponyboy, and the rest of the Greasers because she's a good person, and she believes in doing what is right. Additionally, she feels guilty and partly responsible for what happened to Johnny and Ponyboy.

She said she felt the whole mess was her fault . . . and would testify that the Socs were drunk and looking for a fight and you fought back in self-defense.

Cherry is sick of the violence between the Socs and the Greasers, and she knows that her boyfriend is a part of it. Additionally, she knows that her boyfriend is at fault and that Johnny was acting in self-defense. Her overall aversion to the gang violence is nothing new either. Readers already see her head off and end a brewing fight by getting into the Soc car at the movies.

"No!" Cherry cried. "Stop it!" She looked at Bob. "We'll ride home with you. Just wait a minute."

Finally, Cherry wants to testify on the Greaser's behalf because she has developed a relationship with Ponyboy. It's not a romantic relationship. It's a relationship of mutual respect. She understands that Ponyboy is a dreamer that watches sunsets like her. Cherry understands that the Socs and Greasers aren't all that different, and she wants to stand up for the people she knows to be in the right this time.