Block trades are large-scale, secretly negotiated trades or transactions. For a transaction to be discussed and traded outside the main cryptocurrency market, it must reach a particular volume level.
Normal trading systems do not support block trades. They are not reflected in the overall order book as a result. Usually, these deals take place over the counter or in dark pools (OTC). The trades are set up primarily for two purposes: to protect the confidentiality of buyers and sellers and to control the detrimental effects of sizable transactions in the open market.
Whales on the cryptocurrency market who want to exchange a lot of crypto assets favor block trades. They would be in a poor position in the market if they traded such a significant amount of cryptocurrency on the open market due to slippages affecting their trades.