Another advantage of breathing a tank dry under controlled conditions is to verify the accuracy of your SPG.
Most SPG's are calibrated at the middle of the 0 to 3000 psi range. This serves to minimize the cumulative error at the high and low ends of the scale but perhaps more importantly ensures the guage is most accurate in the middle the dive, - where the diver relies on accurate information to decide when to turn around and start returning to shore, the anchor line, the exit point, etc.
But what this often means is that a 200 to 300 psi error at the low and/or high ends of the scale is common. With regard to the low end of the scale, SPGs that read something higher than zero when the tank is empty are easy to spot and never get out the factory door. What is less obvious and much more common is an SPG that will read zero when there is still 100 to 300 psi left in the tank.
Normally a diver never lets the reading get that low on the dive as they will exit at 500-600 psi. But it is nice to know that a 200 or 300 psi pad exists for those rare occassions where you may be sharing air or wanting to finish a full 5 minute safety stop in a situation where you have less than 500 psi showing on the SPG. Knowing that as soon as the needle hits the peg means you still have a couple hundred psi and a few more minutes at 15' will let you take advantage of those few extra minutes. Conversely, if your SPG really means zero when it says zero, knowing you better be at the surface when that occurs is important too.
In my experience, electronic SPG's and the SPG functions of air integrated computers show greater resolution (5 or 10 psi displays) but they are usually no more accurate at the bottom of the scale. Overtime, they are also more likely to develop calibration issues than a mechanical SPG.