I do not know the context of this question. But I believe that it reflects a very old view of router configuration. Many years ago most Wide Area Network connections were based on serial interfaces. The serial connections were effective over longer distances, while Ethernet was limited about the distances it could operate on. In modern times the technology has changed, especially in terms of Ethernet for long distances. So it is common in modern network configuration to use Ethernet for WAN.
HTH
Rick
