It allows cultivators to simulate other types of law powers in order to make better use of their immortal treasures. It uses spiritual sense as a medium to use one type of law powers to simulate another. The True Mantra Spirit Conversion Technique couldn’t allow one to perfectly replicate a specific type of law powers, but it could certainly produce a very similar product.
Generally speaking, it was quite rare for a cultivator to obtain an immortal treasure that just so happened to correspond with the law powers that they were cultivating, unless that immortal treasure had been refined specifically for them. As a result, cultivators were often unable to unleash the full power of the immortal treasures in their possession. A powerful cultivators in the True Mantra Sect had invented this True Mantra Spirit Conversion Technique to remedy this exact problem