The 'Seaman's Book' provides a travel document that can be used instead of a passport in some cases. In order to be used as a passport, the situation must be appropriate (i.e., joining or leaving a vessel) and the book itself must contain all of the information that a passport would normally contain.
But the big selling point of having a Seaman's Book, comes when it is presented by a person who would ordinarily require a visa to enter the country. If the seaman can show that he is joining or leaving a vessel then the border official can waive the visa requirement.
Example: A Nigerian national with a Seaman's Book is joining a vessel currently anchored in Marseille and due to sail somewhere else. He can present his Nigerian passport along with his Seaman's Book and enter the zone without having to get a Schengen visa.
This will work in every country that is a signatory to the International Labour Organization (ILO) Convention 108 (and in limited circumstances ILO No. 185). The linked site contains all the provisions and details.
So they are a great thing to have, but hard to get.