That sizing makes it easy to use "Shark Bite" type fittings on PEX. Quick and easy. It seems Home Depot has more SB style fittings in Metal, while Lowe's has a better selection if you want Plastic. Also noticed Amazon has a decent selection. I was looking for Plastic, but I'd imagine they offer metal too.
I've used plastic SB style fittings on PEX in our RV, and metal in our boat. No leaks from either.