Because there’s a bunch of legacy wiring out there that doesn’t have a ground wire, only hot/neutral. So if you’re designing an appliance that has to work with

Because there’s a bunch of legacy wiring out there that doesn’t have a ground wire, only hot/neutral. So if you’re designing an appliance that has to work with these two-prong outlets, especially if it has a metal housing, you want to know which wire is the “shock your customer” one and which is the “probably don’t shock the customer” one.