Why the distinction between machine and work coordinates matters
A CNC control always knows where it is in machine coordinates, measured from a fixed home position established at power-up, but a program is written in work coordinates, measured from a point on the part itself. Every G54 through G59 work offset is really just the machine coordinate location of that part-zero point, and confusing the two is a common source of programs running in the wrong place.
How the numbers are calculated
Work coordinate position equals the machine position minus the work offset for that axis. Since the work offset stores the machine coordinate of the part origin, subtracting it from the current machine position tells you exactly where that position sits relative to the part rather than relative to the machine's home.
Practical tips
Use this the opposite direction as well when needed: adding a work coordinate value to a work offset returns the corresponding machine coordinate, which is useful when a control read-out shows one coordinate system and a problem needs to be diagnosed in the other. Always confirm which work offset is active before doing this math, since a program running in the wrong offset will produce numbers that look plausible but are measured from the wrong part-zero entirely.