Each time a match-4 or greater is made, an item is created in place. These are intended to speed the game up. Skills need to be placed on marked cells at the expense of harvested gem elements, so an explosive for instance, effectively clears cells while harvesting resources in one go. Even better, an item can trigger others as a chain reaction if lined up properly.
The type of item created is determined randomly. All unlocked items can be added to the range of possibilities. Perhaps for strategy, opting only for one particular item would be suitable. It all depends.