احْفَظُوا الْكُتُبَ وَلَا تُهْمِلُوا أَوْرَاقَهَا. فَحِفْظُ الْكِتَابِ حِفْظٌ لِلْمَعْرِفَةِ.
Protect books and do not neglect their leaves, for protecting a book protects knowledge.
The first statement gives a concrete instruction; the second reinforces it through a more general principle. In that relationship, the final independent assertion has a tadhyīl-like function, embracing and strengthening the intended purpose of the instruction.
Not every closing statement counts as tadhyīl. It must genuinely reinforce or encompass the earlier meaning. Here the clause can also explain why the injunction matters; rhetorical functions are not always mutually exclusive.