People who want to check the scale. A line on the side of a container doesn't generally get out of calibration, and a glass beaker, for example, will have obvious signs of damage if its weight has changed more than a miniscule amount. Thus, if you know the starting weight of the beaker, clean it properly and fill it with a known volume of pure water, you have a check weight accurate enough for most people's purposes. Anyone needing more accuracy than that probably already has a set of standards to calibrate their instruments with.