You use it to repeat something you have already said because the listener is not taking it in, as in the flat refusal meaning I said no, or the line insisting that they really cannot sing the latest songs. It carries frustration or urgency and often has 진짜 nearby, as two of these lines do. It is built from the quoting ending plus the because-marker, so it literally means because I said so. The polite version is 다니까요.
Register: casual and emphatic, sometimes irritated