If sealed within the body of low grade demon beast, it could forcefully raise its growth speed. However, this causes them, and their offspring, to become extremely brutal. The Scarlet Ghosts the Nie Li and his friends face in the Ten Millennium Spatial Array appear to have been produced using this method.
As for humans, once the impurities are removed, a Blood Crystal can be used to aid the rise in cultivation and greatly enhance soul force. For this purpose, Nie Li killed hundreds of Scarlet Ghosts and then purified their Blood Crystals, until he had enough to give everyone in his group one.
The Blood Crystals most likely came from a nearby mine that is controlled by the Silver Winged Family. The miners are slaves. Nie Li was able to get a large amount of blood crystals from the Silver Winged Family, allowing him and end the rest of his group to break into gold rank.