Why is this the answer?

The answer is piano because this beautiful riddle uses clever word pictures to describe one of the world's most wonderful musical instruments. A piano is one of those things that is simple to look at but very complicated and amazing on the inside. Let's look at the first part of the riddle: "I am a box that holds keys without locks." When you look at a piano, especially a grand piano, it certainly looks like a big wooden box. This sturdy container is what holds all the magical parts. The "keys" it talks about are the long row of white and black buttons you press with your fingers. There are usually 88 of these keys on a full-sized piano. These are special keys. They are not like the metal keys you use to open a door. Instead, when you press them down, they make a little hammer inside the wooden box hit a tight string. The string vibrates, and that is what creates the beautiful musical sound. Now for the second part, which is the most poetic clue: "yet they can unlock your soul." This is a tricky part, because it does not mean the piano literally opens your chest. It means that the music a piano makes can create very strong feelings inside of you. Think about how a fast, upbeat song can make you want to dance and feel happy, or how a slow, gentle song can make you feel peaceful and calm. Music has a special, deep power to make you feel all kinds of big emotions. When a musician plays beautiful music on the piano, it feels like it touches the deepest, most special part of a person, which the riddle calls your "soul." That is why the piano fits the riddle perfectly. It uses a long row of simple keys to make powerful music that can change how a person feels.