PhD thesis Report has questions from reviewer
My experience is the US, not the EU, but, while the "defense" is mostly pretty pro-forma, you still need to give a reasonable answer to any question asked there. In rare cases the candidate failed the defense and failed to get the degree for responding to a question wrongly when they "should have known" the answer. 

I think that the written questions are given as a favor to the candidate, though the questions will most likely be only about the dissertation rather than general questions about the field as a whole. In some places you need to be prepared for both. But if you can answer the written questions you have a good idea about what a careful reader of your work might still want to know. This prepares you for the first, but not the second sort of questions - about the field as a whole. 

The lesson here is not to focus so closely on the dissertation itself that you forget everything else you know. Freezing up can be an issue for some people, so it is good if you have practice in speaking generally (say through teaching) and have ways to calm yourself quickly if asked an odd question. This is more of an issue in those places that have an "out of field" member of the defense committee - a person from a totally different field who will not have read your work. Your answers need to be reasonable, even when not strictly correct.