What does 'holding zero' mean and why do reviewers care?

Holding zero means the caliper returns to a true 0.000 reading when the jaws are fully closed, even after being turned off and back on. Reviewers across r/Tools and r/reloading single this out as the single biggest difference between cheap and quality calipers, because a tool that drifts off zero forces constant re-calibration. iGaging's Absolute Origin design and Mitutoyo's Absolute scale are praised specifically for retaining their zero reference.