"All of a sudden" means "suddenly". I think you are better to think of it as a fixed phrase. I guess that it is sometimes used because it sounds a little better, or because it is easier to follow if you miss hearing the first few words?

"All of a sudden" generally means something happened unexpectedly, very fast and seemingly without warning or notice. “All of a sudden the husband comes.” = The husband came unexpectedly without warning or notice, and it seemed to happen in an instant. He wasn't there and now suddenly he's here.