When singing, you use your ears to match the pitch and opposed to keys, position, etc. You use your vocal chords and basically your body as opposed to carrying an actual instrument. Singing is often more natural because you can use your voice to change the pitch. It is also more unique to each person because there is more contrast between singing voices than there is with different people playing an instrument. In addition to this, it is more unique sounding because your range might not match everyone else’s. Instruments generally have a specific range of how high an low they can hit, but voices are individual in this sense. :)