Why was the invention of writing such a big change for the world?

For most of human history, every single thing anyone knew lived inside a head. Stories, recipes, who owed whom a goat — all of it floating around in soft, forgetful brains. And then, a few thousand years ago, somebody did something sneaky: they pinned a thought to the outside of their head. They wrote it down.
Before writing, your memory was your whole library. When you forgot something, it was just gone — like a sandcastle at high tide. And when a person who knew a great deal grew old and died, every fact they hadn't passed on by talking went with them. The world kept losing its smartest libraries, over and over.
