To use an analogy: I'm a medical student at the moment and I've got to wait six years until I am qualified and certified to diagnose a patient and solve their problems. However, I work as a nursing auxillary to pay my way through medical school and I'm allowed to diagnose minor problems like bed-sores, irritation from cannulation, poor oxygen saturation and treat them (apply dressings, remove and replace cannula, administer oxygen) despite not having any qualifications.

I think that summarises IT certification too: for important and critical systems like air traffic control, health databases, electrical distribution systems, chemical manufacturer systems etc, certification is required to show that you've not only proved you have the skills to prevent life-threatening problems but to solve them. It also provides a measure of legal protection, both for you and for your employer should something go wrong. However, for minor systems like B&M store systems and other "low-level" systems there's no need for certification if you've got the skills, howsoever obtained, to run and maintain them.