Well basically you want to use it when you want to keep track of a list of things that don't exist as objects. For example, scores, coordinates, colors etc. Now we have other objects that can do that, but SQD offers a different way to access that data on the fly, and either use it, or change it without having to worry about preserving order.
A good example might be powerups. Say you have a ship that gets a powerup when it collides with an object, but you don't want to use the bonus right then, like only when a player presses a button.
With Stack you can do that like a bullet clip, where the first bullet in is the last one out.
+> Ship on collision with bonusbomb
->stack push bonusbomb
+>Ship on collision with bonusfireball
-> stack push bonusfireball
+>M&K on space bar pushed
->Ship spawn object(stack.top)
->stack pop top
So with that if the ship ran into a fireball, then a bomb, and then another bomb, when you pressed space it would spawn a bomb, then another bomb, and then a fireball, but then the stack would be empty, just like a clip.