The advantage to a drinking vessel being transparent is that you can see the contents. Redditor toper-centage takes advantage of this by nesting ordinary plastic cups, then using them like animation cels:
Clever, creative and low-cost entertainment!
"I cope with frustration and stress by drawing [on] cups," the artist writes on Reddit. It's presumably job-related stress: The cups, he says, come from the water cooler at work.