I'm thinking especially pre-20th Century.
How did the formations wait?
Were they all drawn up nice and neat, like we do on a table? I'm thinking how did they 'Ablute', or eat or what ever?
I know in some Napoleonic battles, some battalions would be in position for hours - did they just stand in formation from Dawn to mid-afternoon?
At Waterloo the British sat on their packs while waiting, then stood too.
The Zulu reserve (lions) would often sit with their backs to the battle.
The rationale was to prevent an impetuous rush into battle, and remain under their commander's control.