Market orders allow traders to directly access an order book's liquidity by buying or selling into the closest sell orders or buy orders. When creating a market order, traders size their order by enterin the amount in Euro in case of a buy or the amount of the crypto asset in question if they want to sell. Unlike with a limit order, entering a price is not necessary. The order is automatically executed at the best price available.
Usually, a market order does not show up in the order book because it is executed instantaneously. This results in a market order only being visible to other market participants after the fact in the trade log which is constantly tracking all executed trades.
A market order results in a higher taker order fee as it depletes an orderbook's liquidity instead of adding to it.
Market orders have their use nonetheless: They are usually executed instantaneously while limit orders, in theory, can remain in the book for a long time without being executed. This can be especially important when price is trending or exhibits high degrees of volatility. Market orders also are an element of stop market orders which are used for risk management.