If you go on a det to another unit, you'll receive a Mess Bill from them. If you dont warn out of your parent Mess, you'll receive a bill for that mess too.
The logic is that by warning in/out, you only get billed for the time you are on that particular Messes Ration strength.
Warning out at weekends serves no purpose whatsoever. It may have done once upon a time, when the Mess Manager would be working out how many needed feeding, but not any more.