Other than having proper identification on hand if stopped by authorities, are there reasons to carry my US passport when I'm outside in Korea? I don't think I would get ID'd for drinks since I am 25 and the drinking culture is not as strict as the US.
The other answer seems good but not specific to South Korea. Here is my 5 years experience as an expat in the country:
Except if you are driving or if you are participating to troublesome situations (fight, illegal activities,...), the police will never check your ID (contrary to the neighborly Japan, where random controls in the street are common).
However, there are situations were you will need your passport:
- Banks will ask your passport for some operations like opening an account, international transfers, etc... Note that; as an expat, I have an ARC (Alien Registration Card) and, even though I have been doing the same operations in the same branch with the same agent for some years, they continue to make a copy of my passport.
- Not sure of it, but you are likely to need your passport to buy a cellphone or a SIM card.
- Some train passes are reserved to foreigners, so you will need your passport to get them. Also, you need to give your passport number when buying train tickets online, and it will be checked when taking your ticket at the counter.
- ID controls in bars and restaurants: some bars systematically check IDs of all customers, even if they obviously look adults (I am 32), and bouncers may refuse you to enter if you don't have it. Any ID that looks official and show your age is usually valid though, not only passport.
- There are a lot of duty-free shops in big cities. They will check your passport and plane tickets.
Again, all of this is from experience and YMMV. Some people may be flexible and accept any ID (I entered in a ID-checking bar showing my age on FaceBook), other much less. I don't know if a copy of your passport is a valid substitute of your passport.
From personal experience: I never carried a passport with me on the domestic trips - including the same-day land trips around the country - anywhere I traveled, and I've been to over 50 countries so far. Only two times I was asked by police to show the passports, and both times they let me go after I said the passport is in the hotel safe. They only asked the hotel name, and lost all interest after that. I do carry a photocopy of passport with me, which is often asked by merchants when you purchase something with a credit card.
The could be more reasons, depending on how you spend your days:
If you are in a different part of town, meet someone, have a good time with him/her, and at some later point decide to rent a hotel room - the hotel might not check you in without your passport. In Korea they WILL not check you in, but in other countries like Philippines they would - as long as you look over 18. If you're Caucasian, please mind that in Asia it is difficult for many locals to estimate the age of Caucasian people, so please don't get offended if they assume you're under 18 and ask for ID.
If you decide to purchase something relatively expensive, the merchant might want to see your passport, and might not be happy with photocopy.
In some countries (notably Singapore) you need a passport to buy a SIM card, so if you're walking around and suddenly decide to buy one, you won't be able to. A photocopy will NOT work there (but will work, for example, in Malaysia).
If you want to purchase some duty-free stuff, they usually need your passport as well, to record the passport information as well as ensure your visitor status.
If you got into medical trouble - for example, you broke your leg and need to be admitted to the hospital - the hospital will want your passport too before admitting you.
And of course if you get questioned by the police - and for that being in close proximity of some fighting drunks is enough, or sometime they just raid the whole club - having a passport might make a difference between going home right away, and spending a few hours in a police station/detention cell until they verify your identity and legal status.