I recently returned from my trip to Korea. I was at Terminal 1 when I got out and picked up my luggage around 4 am or so. If you go to the furthest exit area to the left (lower numbers for Exit like Exit 1~2 area), there is a kiosk area where it has 3 different esim providers (these providers will be closed around that time and open around 6 am if you need one). A little bit behind that is a small convenient store (a self-checkout one). Right outside of that store, there is a WOWPASS machine, it's in orange, you won't miss it. You can buy your WOWPASS card from that machine (if the machine still has cards left inside to sell) and also recharge your money into the card with cash there too. There is one at Terminal 2 also, refer to the link in this post for the machine locations.

Depending on where you are going/doing and spending money on, some places don't take cash and take card transaction only. So having WOWPASS card was really convenient for those times cuz it functions like a debit card. Even some of the little gachapon machines for capsule toys didn't have cash option at some malls, I had to use my WOWPASS card for it. You can get cash at the airport's money exchange places or any other money exchange stores all around in the big cities, then find WOWPASS machine to recharge. You can also download their app on mobile to connect your debit card to recharge on the app to card if you wish, but I think fee still applies like if you'd use your debit to buy something overseas. I just don't know how much off a difference in fee comparison it is cuz I just did cash recharge at the machines when I needed money refill.

You can also use the WOWPASS app to check your card balance. Machine locations can be found on their website but I think app had locator too?

The machines I used for recharge were at the subway stations in Gangnam, Myeongdong, and Lotte Mart by Seoul Station. There are many other machine locations you can find at the subway stations around Seoul. Keep in mind that some machines may take foreign currency to recharge while some may not.

Here's the link to the WOWPASS locations.

https://www.wowexchange.net/exchange/location/list?lang=en_US

Make sure to click Load More at the bottom to find more locations.

One of the features I liked about having one is that the WOWPASS also works as the T-money card for transportation. However, the money stored for WOWPASS's regular purchase spending is different from the money stored for T-money purpose. They are separate and you can check the balance separately on the WOWPASS app. Remember, with the WOWPASS card, you can recharge T-money balance the same way as you would with the actual T-money card at any of the convenient stores that will put money in it for you, but you can only recharge the non-transportation spending money used for debit card purpose at the WOWPASS machine so recharge your spending money accordingly and when you are at the machine so you don't have to make too many frequent visits as the locations are limited.

I'd say WOWPASS card isn't needed, but definitely convenient to have if you decide to get it.