“What is the use of the utuja kaya?”
I am not sure whether the utuja kaya has a “use”. It just arises due to the cittja kaya. As I said, citta arise and fade away quickly. Part of the energy in those cittas are released as an “electromagnetic wave” to use terminology of modern science.
These days, there are machines that can monitor this “aura”. It is an energy field.
– When one dies, this field is turned off and the “dead body” becomes inert like a piece of wood. So, this “aura body” is a sign of life.