how does Orwell show that to be literate, to be able to read and write??

The commandments are changed by napoleon at his will. The animals need to be able to read to understand this. They need to be able to write to express themselves. Clover has to ask Muriel to read the commandments again because Muriel thought sleeping in beds was not allowed. The animals must be able to read and write to avoid being brainwashed and manipulated by pigs who can.