Immense Danger.

The vestige of a Celestial is very expensive to make. It is also easily destroyed. It is like an expensive consumable but it has the advantage of being concealed and controllable from a distance. The tree father didn't need to worry about the expensive cost of divinity since the gods were generous with divinity when they exchanged it for their demigods. So the vestige was a good idea to stay in touch with the GodSlayer without being close to it.

The tree father chose this plan of action because Legion believed that if he can find out about the GodSlayer from some random demigod that he kidnapped, the odds are that others have found out about it and it is not a secret. He could have destroyed it but the weapon was developed to harm the Celestial Supreme so the tree father decided to render assistance if it is used against Zernon or destroy it if it is ever used against Legion.