Everybody has dirty data. If you are lucky, on a normal business day it isn’t noticed very much, if at all. But, during a data migration, it can cause more than an embarrassment. It can cause a whole implementation project to come to a stop. Or, in extreme cases, it can make the whole dataset unusable.
Business data is valuable. Data is wanted for the convenience it brings. And, data is needed for legal reporting requirements. We can’t just throw dirty data away. We must respect the body of knowledge.