Why was it important for Frederick Douglas to learn how to read while he was a kid?

From the text:

I now understood what had been to me a most perplexing difficulty‐‐to wit, the white manʹs power to enslave the black man. It was a grand achievement, and I prized it highly. From that moment, I understood the pathway from slavery to freedom. It was just what I wanted, and I got it at a time when I the least expected it. Whilst I was saddened by the thought of losing the aid of my kind mistress, I was gladdened by the invaluable instruction which, by the merest accident, I had gained from my master. Though conscious of the difficulty of learning without a teacher, I set out with high hope, and a fixed purpose, at whatever cost of trouble, to learn how to read.