There many “limited time only” sweets and snacks in Japan. For example, there are sweets of a chestnut flavor and they are sold only in fall as fall is a season of chestnuts. However it’s not always a seasonal flavor. Some limited time only sweets/snacks have a unique flavor like goya (bitter melon), squid, and so on.
I guess Japanese people like to try those as markets always sell limited time only sweets/snacks every year. But it’s a game plan for markets to sell many products in a short term.
Many people from other countries are surprised because they see so many limited time only sweets/snacks in Japan. You will definitely see limited time only things when you come visit Japan.