Ever hear that saying, "everyone has a plan until they get punched in the face?" Well, it's the same on the battle field. You can come up with a lengthy order, only to have something change and you need to UPDATE your Marines that something has changed.
A Fragmentary Order, or "Frag-O" is an abridged order. It usually contains an updated Mission and Execution issued by higher. It is then your responsibility to then update your own Mission and Execution and issue the Frag-O to your Marines. This was a big point of stress for me at OCS. Looking back, it might seem like the whole plan goes out the window, but relax. Most of your order will stay the same.