It was used for ducking but its more direct purpose was hiding. There would be random boxes that were empty throughout each level so it was a good hiding device but the better players used it to break the opponents auto-lock or dodge a headshot.
In the Metal Gear Solid series Cardboard boxes are used for hiding. In this game there are random empty boxes scattered so hiding in one is an ok hiding place but what the pros used it for was to duck headshots, break AL, and run up hills faster.
But check out the ownage by Chuck138! I had a broken thumb and was in a cast at the time.