Cookie cutters make plain dough into fun shapes. If you're decorating a cake, these shapes can help place sprinkles and icing.
Cookie cutters have many uses in the kitchen, including shaping hamburgers. If you have a favourite shape, use it as a template for icing or sprinkles.
Put it on the cake and then just put the decorations inside. If you're trying to create a specific shape and don't have a cookie cutter for it, make your own.