Why QRL? is the standard Q signal for asking whether a frequency is occupied: "Are you busy?" or "Is this frequency in use?" Sending QRL? (and listening) before calling CQ on CW is basic operating courtesy, since the station using the frequency may be weak or working a DX station you cannot hear. A reply of "QRL" or just "C" (yes) or "R" means move along and find another slot.
Watch out The choice about keeping the frequency clear is closer to QRX or a simple request to stand by, and full break-in is QSK, not QRL.
QRL = "aRe you Loaded/busy?" Ask QRL? before you call CQ. QSK is break-in, QRL is busy.