At school we are taught to write properly. Our work is marked and graded. Grammar matters. And so on.
But when we talk with our friends, we don’t worry about that stuff.
We’ll talk in fragments of sentences. We’ll start sentences with the word “and”. We’ll end sentences with prepositions.
We don’t worry about the structure of our sentences or paragraphs. Not that we really speak in paragraphs.
The point being, when we communicate through writing, everything is more formal, more structured, more correct.