"This move will save you so much time at the security line. If your carry-on suitcase is just full of clothes, you can throw it on the security belt while you focus on your personal item, which will have all the "questionable" items like  liquids, large electronics, and food. 

Packing this way also ensures everything you'll need in-flight will be in your personal item (which should be stowed under the seat in front of you). That way, you won't need to get up and open the overhead bin to rummage through your suitcase mid-flight."