وَأَمَّا الْإِبْدَالُ مِنْهُ أَيْ مِنَ الْمُسْنَدِ إِلَيْهِ فَلِزِيَادَةِ التَّقْرِيرِ
As for substitution from the subject, it is for increased confirmation and establishment.
In badal, a second dependent expression re-presents what the speech is directed toward. In whole-for-whole substitution, the second element is particularly prominent as the intended target of the grammatical judgment.
This is not empty repetition: the listener is guided from the initial expression to a more settled understanding of the referent or relevant portion.