what is the metal pin for

It is a SIM removal tool. It is convenient to have should you ever need to change the SIM. Paper clips also work.

My iPhone XS Max is the very first iPhone I've received WITH a sim tool in the box. When I opened the box and saw it, I was like oh, so that's what people mean when they ask about one in the box. I've just used paperclips if I've ever had to remove the sim.

Which is good because I've lost 3 SIM tools within days of getting new phones 😉

I used to have bags of them (mostly from HTC) back when I worked for Verizon.

Generally, in the U.S., if you purchased a phone that included a SIM for a particular carrier, there was no SIM removal tool. They were only included in with the unlocked phones.

That makes sense. I bought an AT&T phone, but it's not locked as I paid for it upfront.

I remember people complaining that Apple had somehow cheated them by not including the pin. Perhaps they're just putting them in all the boxes now.