When light of virtue condensed, it would slightly increase the mental energy and physical strength of the cultivator affected. Song Shuhang learned about this when he saw the foreign monk using the soul ferrying technique to accumulate virtue.
At that time, he was still in the body-tempering stage, and he needed to strengthen his physique. As such, he had been rather envious of the foreign monk's soul ferrying technique. This was also one of the main reasons why he decided to learn the soul ferrying technique later on.
Afterward, with the improvement of Song Shuhang's strength, his mental energy and physical body continued to grow as well, which caused the enhancement brought about by the accumulation of virtue to be negligible.