Geode/NZ acquisition boards are self-calibrating to remove DC offset. This maintains the dynamic range of the system and allows simultaneous measurement of small and large signals. Calibration takes approximately 2 seconds. There are some situations when it is desirable to defeat the calibration, for example if you are stacking very quickly, or if you are doing marine surveys where the inter-shot times need to be small.
By choosing to calibrate every nth save, you can ensure periodic measurement appropriate to the kind of survey you are undertaking. Also by keeping the temperature of the instrument stable, you can minimize the need for calibration.