By having your claim confirmed by anindependent control body:
Your credibility increases
Certification lets you stand out from the crowd of self-claimers.
And it ensures fair competition between trustworthy players who, incidentally, are largely in support of our certification marks.
Moreover, although you have nothing to hide, you would also prefer your precise product composition not to be disclosed or leaked. You want the control body to have a policy of treating all data 100% confidentially.