An auditor suggested it during our initial audit after we showed him email records of the labels. Mostly it's a CYA for us.
Our process now is:
The customer sends us a label sample via email.
We print on our label printer and scan it so they can check and approve via email that the print job is accurate.
We print the email approval and compare the printed label and their original email to make sure it matches letter to letter and number to number.
We stick the printed label to the email copy (or staple if not enough room), then whoever does the verification signs and dates the page and makes sure part of their signature is also on the label.
We keep these copies for 3 years (shelf life ranges from 3 months to 2 years so we keep them together by calendar year so we don't have to stress about accidently shredding something we shouldn't). We also haven't done a whole lot of relabeling until this year, we've had more projects so far than we have the last 2 years combined.
Some products contain allergens, but our customer does not mark their finished products outer case packaging as containing allergens (just says milk chocolate or dark chocolate w/almonds). Per the auditor suggestion, we also verify via email that they do not want the allergens listed with a "contains" section on the outer case. We build variety packs in a plain brown box with the finished product inside and all that information is on the primary packaging labels.
It's all to protect us from getting an NC because we can show proof we are following customer requirements.