If you work as a community or liaison interpreter and you live in a country that has a certification exam you absolutely want to take that if it is given in you language. To be a certified interpreter gives you a professional label and it is of course a quality label. You have actually proven that you meet a certain professional standard.
Most exams are criticized for different reasons depending on the exam. The main reason for this is that interpreting is notoriously difficult to measure and test. But no matter whether the test is a 100 % apt or not it is still testing your skills and having passed it is a perfect way of promoting yourself.
In Norway the next certification exam this time is given for Bulgarian, Sorani and German. More information here.
Thank you Anne Birgitta for the tip.