It is connected to the knowledges and insights which the Prophet was given and by which he was preferred. When something like this comes from a man who could not read or write, did not study and was not taught, it provokes amazement and profound consideration. This is a miracle for a mortal man and there is no imperfection in it since the goal of reading and writing is to gain knowledge. Literacy is a tool for this and a means of gaining understanding, not an end in itself. If the fruit and goal of something is obtained without them, there is no need for the means and cause.
Illiteracy is a fault in others because it indicates ignorance and is a sign of stupidity.
Glory be to the One who made the Prophet's affair different from other people's and placed his honour in something that for others is a fault, and his life in what would have meant death for others. He had his heart split open and what was inside of it removed, and that was the perfection of his life, the extreme strength of his self and the firmness of his heart. For other people, it would have resulted in their destruction and death. It is the same with much of what is related concerning him and his life - about his making do with little of this world, his clothes, food, transport, his humility and employing himself in his business and serving in his house, his doing without and turning away from this world. Both the trivial and important things of this world were the same for him since its affairs pass away swiftly and its states are overturned.