The aim is not to trade but to degrade. A burst of messages can add microseconds of processing delay at the gateway and in downstream feed handlers, briefly advantaging whoever generated it.
It is banned as disruptive trading practice, and venues police it with message caps, excess-message fees and order-to-trade-ratio surveillance. It is not a grey area: it is misuse of market infrastructure.
Example: a participant sends 80,000 add-cancel pairs for a single symbol in two seconds. Feed handlers that process 30,000 messages per second fall 3 seconds behind; anyone pricing off that feed is trading on stale data while the stuffer, reading a direct-feed locally, is not.
Related: order-to-trade-ratio, layering, market-manipulation, latency