Why do I care if my phone is SIM unlocked?

Would you buy a car that can only use the right lane? SIM unlocked, or network unlocked, phones aren't a big deal for many of us. We're using a network provider that works well in the places where we spend most of our time (or should be), and using a phone that was designed to work on that particular network. Everyone is happy, everything just works, and we don't take time to think about it. But we should. While we're just fine with the status quo described above most of the time, there are thre...