Why fixture offsets matter
A fixture plate or vise bolted to the table sits at a fixed, known distance from the machine home position, but part zero is defined relative to a datum on the fixture itself, not directly on the machine. Chaining those two known distances together is how the actual work offset value entered into the control gets built, rather than guessing at it or touching off blind every time a fixture is swapped in.
How the number is calculated
Fixture offset equals the distance from machine home to the fixture datum, plus the distance from the fixture datum to part zero. Once a fixture has been qualified this way, the same offset applies every time that fixture is loaded back onto the machine, since the fixture datum to part zero distance is fixed by the fixture design itself.
Practical tips
Qualify each fixture once with a precise measurement, such as an edge finder or probe cycle against the datum surface, then record that offset permanently for the fixture rather than re-measuring it on every job. This is the entire premise behind quick-change fixturing: once the machine-to-fixture-datum distance is known and repeatable, swapping fixtures becomes a matter of loading a saved offset rather than re-touching off part zero from scratch.