I have been providing corporate services here in Spain and a lawyer recommended them.
Nominees only signs the documents but have not real power over the company. You can have an undated shares selling contract & resignation letter signed.
Because Uncle Sam started to arrest foreign bankers that didn't complied with US regulations and their request to share data of all the americans with an account there.
Obviously bank secrecy was a fact then and they weren't allowed to share such data so the best way of not having problems was...