Or to be sure that all involved are focussed, fresh and sober enough to give a decent effort into practicing a parade for HM The Queen?
Having taken part in a few Cenotaph Parades in London and as such also had the required practices at the home of the QCS, RAF Uxbridge I can't say that we were ever confined to camp.
In my experience all such measures do is create resentment for the event they are there to take part in, hardly the best way to motivate the troops.