Why did Persian add letters to the Arabic script?

Persian added letters to the Arabic script to accurately represent specific sounds in the Persian language that do not exist in Arabic phonology.1

Specifically, four letters were added to the script—p, č, ž, and g—by using diacritical marks to differentiate them from existing characters. This adaptation allowed the Perso-Arabic script to better accommodate the phonetic requirements of Persian, as well as other Iranian languages such as Pashto, Kurdish, and Balochi.1