The Hadith Collection description

This EBook is 4 Hadith collections in one and contains over 18,000 Hadith from the following:
1) Complete Sahih Bukhari
2) Complete Sahih Muslim
3) Complete Malik's Muwatta
4) Partial Sunan Abu Dawud

Source material for are from the University of Southern California MSA site

Features:
1) Can activate bookmark, background colors and add to notes
2) Fast search, searching any words or sentences.
3) Long on click copy and share
4) Big and small text sizes and e.t

Question: Why does it require "write external storage" permissions?

Answer: This is if you want move application to external sd card if your internal storage full.