The Windows kernel debugger needs to operate even when some device drivers fail, and for other reasons, the debugger can’t use the normal Windows drivers for remoting the debugger over serial, network, USB, Firewire (1394), so the debugger needs to write it’s own driver and do other hacks to remote itself. In the beginning, serial was the only requirement, but later faster cables were needed, like USB and 1394.

Well, it looks like Firewire is fading away, Microsoft has removed 1394 support from the mainstream release of the kernel debugger, only keeping it in the WDK build. Also, AFAIK, this is the first time the Windows debugger is being built separately like this.

https://blogs.msdn.microsoft.com/windbg/2016/08/11/kd-1394-work-around/