I plan to make carnitas soon with a lovely pork shoulder that wants out of my freezer. I've made carnitas many times before; what's not to love about Mexican pulled pork? I've never used sweetened condensed milk though, and I see that's a very common ingredient. What does the milk add? The recipes that use it only call for a small amount, a tablespoon or so.
This is a bit of a guess - I've only made carnitas without it - but it probably help helps promote nice browning. The tiny bit of sweetness wouldn't hurt either.
Of course, you'll be fine without it too; I didn't know it was a common ingredient and never noticed something was missing. Still could be good though!