Why Amplitude Shift Keying(ASK) is done before transmitting signals through an optical fiber

ASK means converting Digital signal to analog...
ASK scheme is as shown below

Since the transmission of Digital signals is not at all possible due to infinite Bandwidth requirement of digital signals,hence we convert these Digital signals to analog form to make its transmission possible through the optical fiber.
At the receiver end this analog signal is again converted to digital signal for processing at the destination.