Steady state things, mostly, like "ensure that there is always 30 bars of pig iron" or "ensure that ten quarry bush leaves always are available"; if there are too many objects it will stop making them (particularly useful with things like pig iron, which is worthless except in its role in the steel making process, so you need to have
some but not
lots), and as it dips below the threshold it will start making them again. This is really useful to ensure that e.g. there are always barrels available when needed, but you don't blow your entire wood harvest on barrels or bins or what have you; it can also be used to ensure that your idiot brewer does not brew
everything leaving nothing to eat.
If you play one if the mods with multi-stage metal refining, it is even more useful to ensure that the pipeline does not go dry on the one hand but you don't get a glut of pig iron or whatever either.