Why does airport security use numbered baskets for my carry-on stuff to be x-rayed?
It makes it easier to pull a basket from the line. Its likely that those numbers show up on the X-Ray screen so that if the person watching sees something that needs inspection they can say 


  pull basket number 18 


instead of, 


  i think thats the one to pull, it was 4 baskets after the one with the shoes and the laptop. 


There are also ones are available now with RFID tags inside for a similar reason.