Imagine this: you go down to the your local mom and pop mart for your weekly purchase of toilet paper, bottled water, trash bags, beer, candy, or what have you. As you're rifling through your won to see if you have enough to cover it all, the owner tucks an extra toothbrush, fruit, pack of tissues, or a lighter into your bag with a smile. "Service," he says. You smile, nod, and thank him in surprised appreciation as you complete the transaction and head on your way. You've just experienced one of the awesome things about Korea--service items!
Getting things for free was a little startling at first for me. I wasn't used to being given something just for shopping at a store--that's what frequent customer cards were for at home. But here in Korea, "service" is a wonderful surprise when you shop or eat somewhere. (On a related note, the point cards and customer cards here are great to sign up for, just for the discounts you can get!)
Chances are, you may be one of the few (or first) foreigners to visit an establishment, so the owner wants to ensure that he makes a good impression. Or if you go to a place enough times, you will be remembered and thanked as such. The local kimbab places near my apartment and work are always happy to see me and sometimes bring extra food on the side. Other restaurants are known to bring a whole other entrée to your table, not just a side dish or two.
Now, it's not to say you should expect "service" from any place you go to, because it doesn't always happen. But that's what makes it even better when it does. So take it with a smile, a nod of the head, and a "Kamsahamida." And enjoy!