A vague word is useful when you are trying to convey a vague concept, in much the same way as it would be hard to convey the idea that you know that some metric

A vague word is useful when you are trying to convey a vague concept, in much the same way as it would be hard to convey the idea that you know that some metric is between 20% and 40%, if you are not allowed to be vague and are required to arbitrarily pick some number like 38.51% and say that one instead