My work puts me in touch with all kinds of investors, from beginners to seasoned pros.
And it's made me realize that many investors, particularly those who are new to the stock market, are totally unaware that there are different order types - let alone the advantages that come from using them. So they simply call up their broker and tell him that they want to buy a given stock, bond, or ETF.
There's a problem with that. Unless you name a specific price, you're effectively giving your broker permission to fill your order at any price. Of course, you're guaranteed to get filled as long as there's trading activity - meaning buyers and sellers making a market.