There are two main advantages of fibre versus copper cables: higher bandwidth and greater signal carrying distance.
Optic fibres are thin glass tubes that can carry light from one end to the other with almost no losses along the way. Signals can be transmitted by sending pulses of different colours (wavelengths) of light through the fibre. Obviously the use of light means the signals move at the speed of light – a lot faster than electrons through copper; and are immune to the types of electrical interference that can cripple copper based mediums. The high transfer speed and very low losses and interference translate to more data per second – higher bandwidth.
Copper based Ethernet has a fixed maximum cable length of 100m. The main reason for this ceiling is the time it takes for electrons to get from one end of the cable to the other – for cable lengths of more than 100m, the delay will be too high, and transmitters/receivers will time out, thinking the signal has been lost. To use copper Ethernet cables over longer distances, a repeater or Ethernet switch needs to installed at least every 100m.
Fibre doesn’t suffer from the limitations of copper – 2km is usually no problem, and depending on the quality of transmitter and cables, distances upwards of 100km can be traversed.
Fibre is typically a lot more costly than copper cables – not just the cable itself, but the terminations at each end requires special skills and tooling. However for long distance or high bandwidth applications, copper is not an alternative.
Fibre isn’t just limited to Ethernet applications – many other communication types and protocols can be adapted to use fibre, including RS232/485, Modbus, Profibus etc.
When weighing up fibre versus copper, consider the distance and bandwidth required – anything over 100m generally calls for use of fibre.