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Why ever use a market order?
I don't think you're missing anything. Many modern trading systems actually warn you when trying to enter a market order, asking if you are sure that you wouldn't prefer to set a limit. I fully agree with you that it is usually just better to define a limit even 20% higher than just doing a market trade. Let me give you some examples when you still might prefer to use a market order instead of a limit: But even in those two examples a (wide) limit order might just be the safer thing to do.  So, what it really comes down to is speed: A market order has no other criterias to be defined, is thus entered faster and saves you a few seconds that might be crucial.