I think I get it.
I never checked that box but when you commit it auto-checks, and the next time you open it (here anyway) it's still checked, instead of defaulting back to being unchecked. Got it... I think.
I'm not used to opening a dialog to do something and have the dialog be in a bypass mode where it won't automatically do the thing you opened it for, ever, under any circumstance, so that's something kinda new, but again, not a big deal, and I'm sure there's a logical reason for that.
Like I said about VCA's, I don't have to fully understand that, I only need to know how it works to do what I need to do with it, quantize a few notes and keep it moving.
At any rate, I haven't had any issue quantizing stuff in Reaper.