It's for security tray bingo. If you watch carefully, you'll notice the TSO's furtively checking off the squares on their cards. Every so often, one of them will shout "FEMALE ASSIST" which is really just a code word for "FULL HOUSE". (You'll notice a few high fives when that happens).
Games can take quite a long time to complete, because trays tend to go around on the same belt most of the time - meaning that TSOs have to wait to get transfered to a different lane before they can collect all the numbers they need. If you want to do the TSO's a favor, you could shuffle the trays around a little - carry a few from one lane to the start of a different lane. You're sure to be very popular.