Yes! If MK opens at 9am, and you would like to walk down Main Street relatively empty (so great for photos with the Castle behind you!), then yes. Make early ADRs for Crystal Palace (ie, 8am) and you'll be allowed in the park early to get to breakfast.
The PP is correct that attractions won't be open, but makes no difference when you're headed to breakfast. You should have plently of time to finish eating and head out when the park opens at 9am.
It gets you into a closed park.You can't go anywhere really.None of the rides ,fastpass machines or stores or open ,At Mk all the lands are roped off.It's nice for photo ops though.