Certification can be advantageous if an entity has constructed or altered a facility according to a certified code or ordinance. If someone later brings an enforcement proceeding against the entity, the certification is considered "rebuttable evidence" that the State law or local ordinance meets or exceeds the minimum requirements of the ADA. In other words, the entity can argue that the construction or alteration met the requirements of the ADA because it was done in compliance with the State or local code that had been certified.
For additional information, take a look at the following resources:
FAQ: How does the ADA affect existing State and local building codes?
FAQ: Where can I find a complete set of ADA standards for accessible design?