Compared with a copper electric wire, optical fiber allows longer-distance transmission. Optical transmission using a single-mode quartz optical fiber supports a transmission distance of up to several tens of kilometers without a relay.
Secondly, optical transmission is less sensitive to noise. Unwanted signals other than those you want to send are generally called "noise." When you send a signal through a general electric wire, many electric devices interfere with it, causing "noise." A optical signal, however, is free from mutual interference because of its completely different nature.