An attached pronoun cannot stand alone — it joins onto the end of a noun, verb or particle, like the ـهُ of كِتَابُهُ (“his book”) or بِهِ (“with it”). These small suffixes do an enormous amount of work in the Qur'an, carrying “his, her, their, my, your” and the objects of verbs, all in a letter or two.

From the free course The Language of Quran — Easier than English (Book 1) (LoQ1), taught by Ustad Muhammad Arjan Ali.