This is precisely why you should adjust this setting. If you ignore it then there is a strong possibility that you could have large sections of zero grey level pixels in your image (colored blue with the range indicator). The problem with this is that you don’t know if they are actually zero or are somewhere below zero because images don’t record negative values.
It is important to understand what this Digital Offset value is doing on commercial microscopes. The vendor (Zeiss in this case) is calibrating the digital offset slider so that when this value is set to 0 – you should be able to scan an image with no light (no lasers on – just ambient light conditions) and have the baseline grey level for each pixel be just above 0. In the case where the vendor calibration is perfect – you should not need to adjust it further.
This is where you need to take vendor instruction with a grain of salt – because they will all tell you their systems are always calibrated perfectly. 
Users must understand that every system is different – and do not have the same calibrations as another. Because of this – you may encounter situations where when this slider is set to 0, you have massive areas of zero level pixels. This is especially true if the calibration is not done well – or has not been done for quite some time.
Now does this mean we should rush to ‘calibrate’ the offset? Not necessarily – IF you know how to balance the offset as we discuss in our training video – you do not care what the calibration is. And as it can fluctuate with temperature, humidity, and time – it will always need some tweaking for best results.
Remember that you are collecting raw data at this stage - so as @talley stated - you must treat the low end of the dynamic range just as you treat the high end by not having any over-exposed or under-exposed pixels in your final image.