This technique of building up overdubs live and in real time was pioneered in the 1960s and '70s by Terry Riley, Pauline Oliveiros, and
Robert Fripp. They used a reel of tape looping between two tape recorders.
Now it's done digitally, so its possibilities are that much better. It isn't just a gimmick, it can make some great music in the hands of someone who knows what she's doing. In this video, KT demonstrates how she is a master of the technique. This song kicks ass.