It's called a Soul Gem because it becomes the container for a person's essence. This is done simply for the fact that fighting anything is dangerous and in general, the human body is a weak thing. This allows the body to take damage that would normally cripple a person. If a Puella Magi's body is harmed, it can be healed relatively easy with magic this way.

It still belongs to you and only you. I don't want it nor would I ever take it.