"Omiyage" is one of the Japanese culture. It is difficult to translate that word exactly , but we can use the word
souvenir ,or present in English.
For example, when I go traveling somewhere , I will buy something-maybe some swees like cookies -for my co-workers, especially if I had a day off for the traveling, I will DEFINITLY buy something. Or if it was a bussness trip, I will. Needless to say, not only for co-workers, but also for family members and friends.
This is one of the using way of the word "Omiyage".
And when I am invited to somebody's house , especially when it is for dinner, I will bring something. with thanking for the invitation. This is also "Omiyage".
I don't know exactly when , but it has been for long ages , that's why "Omiyage" is culture, like a custom, could be a tradition.
This is one reason Tokyo Disney Resort have been successfull in Japan. There are so many kinds of souvenirs which let us want buy, and I can see many people have very big bags on the way to their home. I heard before the sales of merchandising in Tokyo Disney Resort is so good, compare with Disneyland Paris.
Now as I mentioned before, it is Golden week. Many people buy for "Omiyage" everywhere. Actually here the variety of Omiyage are sold. Yes many kinds! As for me , I like choosing some "Omiyage" with thinking of somebody whom I will give it. Tonight I am going to the airport, if I have a time I will take photos of souvenier shops.