question #1 Sahih al-Bukhari 2532, Book 49, Hadith 16 : What is the significance of freeing a slave in Islam for the sake of Allah?

When Abu Huraira accompanied by his slave came intending to embrace Islam, they lost each other
on the way. (When the slave showed up) Abu Huraira said (to the Prophet), “I make you witness that
the slave is free for Allah’s Cause.”