The main reason that virtual serial ports over Ethernet are useful is due to the limitations forced upon users by modern computer manufacturers. The majority of modern computers have neglected serial ports in favor of the more universally used USB interface. If your computer does have serial connectivity, the number of ports will be extremely limited.
This can pose a serious problem if you use legacy applications that require serial interfaces with which to communicate. It’s not as simple as using a different physical interface on a peripheral device. Without rewriting the code, many legacy programs will cease to operate without serial connectivity. They do not have the ability to adapt to the modified data flow from a USB port.
So using virtual COM ports is a way to protect your investment in legacy serial-based equipment and applications. You can go ahead and buy that screamingly fast new laptop without worrying about the number of serial ports it does or does not have. Simply emulate them with as many virtual serial ports as you need.