Is LinuxCNC just another user interface? Is there any instance when I might need to use it?
Linuxcnc is a standalone controller that runs the machine in real-time. It is a completely different approach to cnc motion control than chillipepper + tinyg/2 system
In short Linuxcnc does everything where as chilli pepper is a drip feeder and a ui and tinyg is a gcode interpreter they are separate. Where in linuxcnc the two are one and to me the machine does what I think it should do more often
As far as instances where linuxcnc is used it has a big compatability advantage when interfacing to industrial systems and it makes more sense to me from a coding side of things. A much better explanation is avaliable on the linuxcnc forums and inside the documentation included in linuxcnc.