So what’s the advantage of an open or unlocked phone?

A main advantage to having an open phone (or unlocked phone) is that it works with any operator’s SIM cards. An unlocked phone gives you more flexibility as to where you can buy your cell phone service. You also have a greater choice of phones since the unlocked ones are not sold by your service provider. You have to pay the full price for that phone, but you do not have to enter into a new contract with your service provider like you do when you get a “subsidized” (i.e. financed by the operator) phone deal at the operator’s own store.
To unlock your phone, you need a specific code depending on your phone model and your EMEI. Some operators provide it but if it is not the case be warned your operator’s condition and your contract allow you to unlock it.