It's one of the few parts of the Common Application that is entirely yours to construct and build, sort of the final frontier of your application.
By the time you sit down to fill out the Common App, your classes have already been taken, your SATs or ACTs scored, and your teachers already know you. Admissions officers across the US will tell you that the personal essay is your chance to tell us who you are, what you stand for, and what is most important to you.
Maybe it'll be an explanation of why you learned to kickbox, or why knitting caps for preemies is so important to you.
I know it's a short block of text, but that doesn't mean you should shortchange it.
Think of your application this way: if it's a portrait, your essay is the face.
The transcript, SATs, recommendation letters: those are the pose, the background, the positioning of your arms.
Show us how unique you are, where you put your energy, what you spend your time on.
These are the things that help us get a sense of you as an individual and as a potential member of our larger Oberlin community.
If it's done right, the short extracurricular activity essay becomes the color.
So here's my advice: write a vivid short answer, and compel me to look deep into the heart of your application.