Can PCs be connected to Fire-Wire ports?

Yes. However, newer versions of Windows don't support networking over FireWire.

https://en.wikipedia.org/wiki/IEEE_1394#Networking_over_FireWire

They can, as long as you've installed a PCI card for Firewire ports into the back of the computer... cost around £15-£20

yes firewire ports used to be more common a few years ago, you could connect scanners, hard drives video cameras and other PCs

Why would they build a Firewire pot into a computer if the compute could not use them?

Generally they are used to connect peripheral devices like scanners, hard drives, cameras, etc.

I am not aware of any way to use a Firewire to connect a compute to a network or to connect two computers to each other.