Writing usually means thinking about spelling and grammar, but in freewriting you can put all that out of your mind.
Writing is supposed to make sense, but freewriting can be incoherent or nonsensical.
Writing is supposed to stay on one topic and be organized, but freewriting can jump all over the place. Writing is usually for an audience, but freewriting is private. Thus freewriting is even safer than speaking, since we almost never speak except when someone's listening. Thus freewriting removes all the difficulties of regular writing. But freewriting is not easy.