Guardsmen who fail the Household Division annual test for walking with appropriate bearing (ie erect rather than slouching) carry these boxes for a number of weeks until their posture is corrected. They contain sand.
A similar remedy exists for mounted troops who sit too low in the saddle.
When you need to know you will get one and find out the hard way!
Sod being the poor fecker carrying it with all those cans in it though
What else do you think they do on stag for 8 hours?
That plus a spare pair of smalls for overnight stay of course