Why hire a certified translator when many people will try to tell you that any bilingual person, or anybody with a bilingual dictionary, can translate? For the same reason that you hire a recognized professional to do anything important.
Most people can change a light bulb or reset a circuit breaker, but if you need any major electrical work done you hire a licensed electrical contractor. Your electrician has the education and the experience to do the job properly and safely, and the trades certificate on his or her wall proves it.
In the same way, your certified translator’s certificate means that he or she has the training and experience needed to practice as a professional. A certified translator may cost you a bit more, but you will have the assurance that your job has been done properly.