I think you and OP are talking about two mostly different things. “Market order” is an overloaded term. One definition is a specific order type that instructs t

I think you and OP are talking about two mostly different things. “Market order” is an overloaded term. One definition is a specific order type that instructs the exchange at whatever price is immediately available. The other is an order that’s priced to immediately execute without resting in the book. By definition all of the former are contained in the latter category. But not vice versa, with the set difference being “marketable limit orders”. That is orders with instructions to not trade beyond a set price, but where that price is set wide enough that we expect the order to cross the market and fill at arrival. I think what you’re saying is why use a pure market order instead of a marketable limit. And there I mostly agree with you. There’s not much argument besides convenience of not having to fill in the limit price. But in terms of market impact, marketable limits are no different than market orders. In fact to everyone else, they look identical in the data feed.