Market orders are executed at uncertain real-time prices. When liquidity is insufficient and transactions are extremely inactive, sharp price fluctuations are prone to occur. You may therefore trade at a price much higher than the current bid price, or much lower than the current ask price and make losses.
For example:
You intend to buy the AAPL, but there is no pending sell order in the market. If there are no restrictions on the placement of market orders, your market order may be executed at any price. If the transaction price of your purchase is much higher than the best bid price at that time, you may face greater losses.