Mary Warren has come to the courts to declare all of these girls frauds. This is a dangerous thing to do, because at any given moment, they could turn on her, and accuse her of being a witch. She knows this, which is why at the end of Act Two, she tells John Proctor, "I cannot! They'll turn on me!" when he demands that she tell the truth in the courts. And, sure enough, that's what they do. It starts when Abby sees her chance to pounce in after Elizabeth lies about John and her affair. She knows that she'd better discredit Mary, or she'll be in trouble. So, she pretends that Mary is a "yellow bird" that wants to "tear [her] face." Mary knows what she is up to, and demands that she stops. And that is when the girls kick in with their mimics of whatever Mary says.
The echoing by the girls adds intensity--if you have ever had a younger sibling do this "copy game" with you where they copy everything that you say, you know how insanely infuriating it gets. It is hard not to just blow a gasket when someone is doing that do you! It pressures you, makes you rethink what you say, and pokes at the nerves. So, that alone increases the drama by adding chaos and a very nerve-wracking situation--we feel for poor Mary. It also makes the pace of the play pick up a bit--the back and forth makes the audience feel tense and out of control, as it does Mary. She caves quickly, turning on John, calling him a "devil's man," because in the course of the mimicing that is occuring, Danforth seems to turn on her, believing the parroting girls over her. The echoing doesn't allow her to get a logical word in edgwise. It brings the conversation down to an infantile level where nothing real can be discussed. This makes Mary unable to prove her case, or argue with any sense, and puts her in a frustrating situation.
For all of these reasons, the echoing adds dramatic intensity and suspense, and was well-placed by Arthur Miller. I hope that these thoughts helped; good luck!