I can’t give you a formal “why” answer as I haven’t studied Spanish, I just know the language. They way I’ve used is it like when someone asks “¿Que dice?” the response would be “Dice que…” It’s sort of just inverting the question and I suppose it has become a common way to phrase things(even when a question isn’t being asked, seems complicated ik lol). But like I said I’m not a Spanish major or anything, I just grew up speaking Spanish. Hope this somehow helps!