It's to properly illustrate that she genuinely doesn't care about Minji any more. Lets critically think about her charcter for a moment. Someone who is carful and meticulous, a type A person, anxious, constantly second guessing herself is recklessly throwing something and leaving without a care.
This is a turning point moment, her leaving it on the ground means she didn't care enough to properly make sure it went into the trash bin. SHE DIDNT LOOK BACK. aka: she's not looking back to Minji.
Anyway, try reading a novel soon or maybe take a Rhetoric and Composition class before leaving comments that don't make sense. Hope this helps.
I was just making a smart-ass joke. Nothing serious haha
She couldn't properly put it in the trash? She just threw it on the ground right beside the trash? Lazy ass