This is another mystical energy along with Dou Qi that appears in the series.
As of this current point in the manhua, Soul Power has ony been shown to be useful for the job of a Receptarier .
There have been many cases show that one does not neccessary need to be a receptarier to train Soul Force, it is just that Soul Force is mainly use most receptarier.
However, it has many other uses not mentioned in the manhua:
Telekinesis: the user can use it to lift objects and control it movement.
Observation/ Sensing: the user can use it to sense others Dou Qi level as well as soul level (assuming that the one being sensed's level is not too large compared to the user own level). It can be as well use to keep track of the opponent movement or determine their position. This is useful when fighting in the dark or the opponent move too fast for the eyes to see.
Tian rank soul level can sense objects thousands of miles a way.