Certification is a means of proving your credibility in anything from language proficiency to print media production control.
When you got that GCSE in French or your degree in astrophysics, the printed certificate confirming your brilliance was your only means of proving you had competence in the field. Those certificates so proudly displayed on your wall mean something to you, your colleagues, family and friends, and for the certifying body which issued them.
As with other sectors the graphics industry has a wide range of certification options, some more robust than others, and mostly they are used to confirm quality control and for marketing purposes. When print buyers send out Requests for Quotation, certifications are increasingly used to weed out the dross. Compliance with or certification to ISO 12647-2 (Process control for offset presses) is increasingly a requirement by big brands.