It's a test of one's ability to multitask.
A way of proving to one's self that no matter how many things you're already carrying, it's always possible to carry one more.
So, the other morning...not day but 6 am at the teacher's butt crack of dawn, I found myself walking out the door with a backpack jammed full and zipped tight on my back, and a purse, paper/file container, oatmeal bowl, water cup and keys all in my hands. I wasn't sure how I was going to open and close the door to the garage. But then I discovered the incredible utility and strength of my pinky finger. With my one little pinky...and mind you the other 4 fingers were already being used to carry or hold something, I opened and shut the door, and I managed to even push the garage opener button on the wall!
Never underestimate the power of "The Pinky!"