Suppose that you are switching to a new system, and you want to know that your key data is tranferred without damage (photos, videos, spreadsheets, documents, etc)?
Or that you want to be able to verify a backup long after it has been made, say next month or next year?
While infrequent, data corruption does happen, and I get related inquiries from time to time from unfortunate users. The more valuable your data, the more important it is to take steps to ensure that data rot or data corruption has not occurred. Causes can include bad media, bad cables, software bugs of various kinds, etc.