It is just a resource! Hello ResEdit.

Another interesting use of bitmap fonts in HyperCard is to do smooth animations.

The other alternatives for animation all have issues - flipping through cards or scripting the drawing tools are slow and limited, 32x32 icons are too small, and XCMDs tend to be buggy.

Fonts let you flip quickly, have arbitrary size, you can easily have multiple animations going at once, and even move your "sprites" around by moving the text field containing them. You can even layer transparent fields on top of each other to make combinations.