A single party controls both sides, so no risk changes hands and no genuine demand exists. The purpose is to make an asset or a venue look active, to qualify for volume-based rewards, or to print a sale price that anchors later buyers.
It is cheap where it is cheap. NFT collections have shown headline sales between wallets funded from the same source, inflating a nft-floor-price, and exchanges have been documented reporting volumes far above anything verifiable. Trading-fee rebates and token rewards for volume actively subsidise the behaviour.
Detection uses clustering: wallets funded by one address trading in tight loops, round-number sizes, and volume that does not move price or show up in market-depth-crypto. Treat volume rankings on unregulated venues as marketing, and prefer metrics you can verify on-chain.
Related: fake-volume, market-manipulation, nft-floor-price, pump-group