I'm assuming this pre-ordering is a way to cut costs by not having to load extra meals to provide customer choice. If you pre-ordered, then they know they only have to load one meal for that pax, lowering the numbers of "extras" required.
They would ideally want 100% pre-ordered so there won't be any wastage.
Its about ensuring you get your first choice of meal. Main meals are loaded at 100% of passenger load (no extras are loaded to ensure choice) so invariably someone will miss out on their choice. Often managed on board by taking orders for staff travelling on an ID ticket last but sometimes there are no staff.