A premium choice is an option within a chapter that you can select for the cost of a certain amount of crystals. This kind of choice is often really exciting and will have a great impact on the story.
The selection of a premium choice will sometimes result in a premium story path that is not possible to reach without making a premium choice. While this may not occur with every premium choice, a premium story path can often lead to helpful items and special bonus content.
Essentially, making a premium choice can unlock a special ending to the chapter or gamebook, bonus content, or give the player access to information or items that are useful later on in the gamebook.