From a Process Control Instrumentation angle, this BIAS above Zero, is to provide additional info about the integrity of the instrument. If an instrument was calibrated 1V-5V = 0-400 gallons per minute (gpm), then, if 1V was measured by the instrument, you'd know that there was 0 gpm. All would be normal.
However, if 0V were measured, then that would be some kind of alarm condition that the instrument has failed or shorted. A properly programmed control system would throw its respective control loop into manual to prevent slamming the valve either closed or open.
In other words, this BIAS from Zero allows the control system an additional, indirect method to know the health of the measuring instrument (i.e., circuit hasn't Grounded or Failed). If you didn't do this then you might not know if 0V were normal or an alarm condition.
Of course, in the old days, we didn't have all the smart instrumentation that communicates much more diagnostic info. :-)
Update: @Transistor has provided some additional insight, which is much appreciated. For what it's worth, I did realize that there was a digital vs analog conflict in my response (mainly due to the highly technical comments/answers). What I was trying to do was make an 'analogy argument', similar to water pressure vs voltage, to impart a possible reason to not 'just use 0V' as a basis. However, I still may have missed the point. @Transistor, I don't have enough Reputation to Comment, so my question back to you: Should I delete my response? I certainly don't want to mislead. Thanks.