Multibuy offers reduce the price of each unit while increasing the amount leaving the shop. Both effects are intended, and the second one is the reason the format persists.
The saving is real and conditional
A multibuy genuinely lowers the unit price for anyone who would have bought that quantity anyway. For staples with long shelf lives, the arithmetic works straightforwardly in the shopper's favour.
The condition is that the extra units get used. A discount on something later thrown away is not a discount at all, merely a smaller loss than paying full price for it.
Household waste is concentrated in exactly the categories that carry these offers most often, which is fresh produce, bakery and chilled goods with short usable lives.
Why larger quantities get consumed faster
Having more of something in the house raises how much of it is used, particularly for snacks, drinks and treats where consumption is not fixed by need.
Portion sizes drift upwards when supply feels abundant, and a pack that would have lasted a fortnight is finished in a week without any conscious decision being made.
The retailer benefits twice, selling more units now and bringing forward the next purchase, which is why these offers cluster in categories where consumption is elastic.
The offers that are not offers
A multibuy is only a saving relative to the single-unit price, and that price is under the retailer's control at the same moment the promotion is running.
Where a larger pack of the same product is cheaper per unit than the multibuy of small packs, the promotion is a comparison against the wrong baseline entirely.
Unit pricing on the shelf label settles this, because it converts every format into the same measure regardless of how the promotion happens to be described.
Why the format is retreating in some categories
Retailers in several markets have moved towards lower everyday prices instead, partly because promotions complicate comparison and partly under regulatory pressure.
Rules restricting volume promotions on products high in fat, sugar or salt exist in some jurisdictions, and their scope and timing differ considerably from place to place.
Lower single prices are simpler to advertise and easier for shoppers to verify, which suits retailers competing primarily on a reputation for consistent value.
What replaced them
Loyalty-linked pricing has taken over much of the same ground, offering a lower price to members rather than a lower price for a larger quantity.
The mechanism differs but the effect on comparison is similar, because the advertised price now depends on the shopper's status rather than on the product alone.
Both formats reward planning. The decision worth making is how much of the item will genuinely be used, before the price of a single unit is even considered.