Why does the OU certify products that are inherently kosher?
If a company requests OU certification, the OU will certify the product even if it does not technically require supervision.
From the company point of view, the OU is beneficial as a marketing tool which extends far beyond the Jewish kosher consumer, as many people prefer to purchase a product that is inspected and certified. If the OU would refuse to certify innocuous products, this would create ill-will or tension between the OU and manufacturers, and would no doubt diminish corporate interest in kosher production.
For the kosher consumer, there is an advantage as well to always purchase certified product. In theory, it is possible for any item to be prepared in a manner that renders it non-kosher. Thus, for example, the OU once discovered that a particular brand of bottled water was pasteurized on equipment used for non-kosher production. While this is a rare occurrence that hardly ever occurs, the presence of the OU symbol insures with absolute certainty that the product is kosher. Even if the possibility of a product being produced in a non-kosher manner is highly remote, the OU visits the production sites of all products bearing an OU to guarantee their kosher status.
It is also true that kosher consumers do not always know what products do not require supervision, and the OU kosher symbol prevails as a mark of trust for the Jewish community.