Optional and additional information
The optional or additional information box is for facts, not another essay. Use it to explain something the rest of your application leaves unclear: a drop in grades and why, a high school that didn't offer certain courses, a family responsibility that took your time, a change of schools.
Keep it short and plain. Say what happened, how it affected your record, and where things stand now. Don't argue and don't make excuses. Give the reader the context and trust them to use it.
If nothing needs explaining, leave it blank. A padded answer costs the reader's time and adds nothing. Some colleges also offer an optional prompt that's really a short essay on a set topic. Treat those as real essays if you have something specific to say, and skip them if you don't.