Why is this the answer?

The answer is mask because this is a very clever riddle that describes all the different ways people use a mask. Let us look at the clues one by one. The first line, "Tool of thief, toy of queen," tells us a mask can be used for two completely different things. A thief uses a mask as a tool to hide who they are so they cannot be caught. But a queen or a fancy lady might wear a beautifully decorated mask as a fun piece or "toy" for a costume party. The next line, "Always used to be unseen," is true because the main job of a mask is to cover your real face. When you put on a mask, the real you is unseen or hidden behind it. Then the riddle says, "Sign of joy, sign of sorrow." Think about masks used in plays or even on Halloween. A mask can be painted with a big smile, showing joy, or it might have a frowning face, showing sorrow. The mask acts as a sign for that feeling. The last clue, "Giving all likeness borrowed," means that the mask gives you a look, or "likeness," that you have borrowed from someone else. You are pretending to be a superhero, an animal, or a character, and that look is not your natural face. All of these great descriptions perfectly fit the object we call a mask.