Why use "OMIT" components on a PCB?
Extra/omitted parts on a PCB can be very useful for

a) Having multiple versions of a circuit able to be built with only one PCB

b) Debugging, when I lay out an RF board I often put an omitted 1k SMD resistor from the line finishing near to a ground pad. Invaluable to solder a 50ohm coax to for probing the signal on the line, with good RF integrity and without damaging the line. A JTAG or RJ45 socket may be handy for digital access to a board for debug, but is not needed in production, it's omitted for cost or because it won't fit in the final production case.

c) You may often see a 0 ohm resistor in series with a power line, for monitoring current, or isolating parts of the circuit. These pads may also have a trace under them to short them in the normal case, the trace can be cut to use the component.