Furniture is presented as a room rather than as a list of items, and the pricing follows that presentation. The set is the unit of sale because it solves several problems for the retailer at once.
The set price cannot be decomposed
A group offered for a single figure gives no indication of what the individual pieces cost, so a shopper cannot check whether the chairs or the table carry the value.
That makes cross-retailer comparison awkward, because rival sets differ in composition. One includes a sideboard, another a bench, and the totals are not comparable.
Where individual prices are published, they usually add to considerably more than the set. The gap is presented as a saving, though the separate prices exist mainly to create it.
Matching removes a decision the shopper would otherwise make
Choosing furniture piece by piece requires judgements about proportion, finish and colour that many buyers do not feel confident making.
A coordinated set removes that risk, which is a genuine service and is why the format persists rather than being purely extractive.
It also raises the average transaction substantially, since a shopper who came for a table leaves with four chairs and a sideboard as well.
Showroom space rewards the format
Furniture retail is limited by floor area, and a room setting displays several products in the space one product would otherwise occupy.
Staged rooms also sell the outcome rather than the object, which is more persuasive than a row of individual items on a shop floor.
The staging carries costs of its own, including rugs, lighting and accessories that are frequently not part of the advertised price at all.
Manufacturing favours the family
Designing a range around shared components, legs, finishes, hardware and panel sizes, lowers tooling and inventory cost per piece.
The manufacturer therefore prefers to sell across the range, and set pricing pushes buyers towards exactly that outcome.
It also means the individual pieces are usually of similar construction quality, so a weak component in one item often indicates the same weakness throughout the range.
Buying separately changes the arithmetic
Where individual prices are available, pricing the pieces you actually need frequently beats the set, particularly if the set includes something you will not use.
The counterargument is availability, since ranges are discontinued and matching a piece later can prove impossible.
For items where an exact match matters, buying together has real value. For everything else, the set is a pricing structure rather than a requirement.