I'm actually not even totally clear on what they are even for.

The same way you can pick a point on amap, measure a certain distance away on the map and "automatically" know how far away to locations are in really life - you can pick a number on a side rule, move a certain distance away and it will "automatically" multiply or divide two numbers.

It will have a small amount of error in it, the same way measuring distance on a map will have a small amount of error in it.