When you calibrate a scale, you are basically recomputing a coefficient. That coefficient is how the scale converts the electrical signal from the strain gauge into a weight.
Typically they use a 10gm weight and so you have two points on a straight line, 0.00g and 10.00g and (simplifying) if the signal is half as strong, the scale shows 5.00g.
If the scale hasn't been calibrated, it will still show 0.00 (that's the tare function), but it will consistently report an inaccurate result.
If the temperature is very different, even the air pressure (for a scale of sufficient accuracy), you need to recalibrate it.
-----Burton
50+ year / Life / Emeritus
ANA member (joined 12/1/1973)
Life member: Numismatics International, CONECA
Member: TNA, FtWCC, NETCC, EveryCountry (online) coin club
Owned by three cats and a wife of 40+ years (joined 1983)
Author: 3rd Edition of the Sample Slabs book,
https://www.sampleslabs.info/