The character ascends to a higher state of being, possibly even becoming a god. It can result from other supernatural causes, but is most often used as a character's fate after they die, in a riff on the idea of a heavenly afterlife. It is mostly a way to pay off a character. Or play off, in some cases. A way to put the character on a bus, though that's not all that it's used for. If the writers should need the character back, it can be very easy to reverse, in which case the character will.