War service records, where they survive, are a bonanza for the genealogist. They document, usually in much more detail than you will find for any other period, a key phase in the person's life. They name a next of kin providing a link to a parent or spouse.
"For King, Empire and Home: Documenting Service in the First World War" will be presented by Glenn Wright at the OGS Seminar in Ottawa, June 1-3. He stresses that Canadian Expeditionary Force attestation papers, available online here, are just one of a number of records in the service files which also contain medical and pay records. About a third of all men who served in the CEF were born in Britain so these are important records documenting Anglo-Celtic connections.
He also deals with records for naval and air forces service, and for those who served in other allied forces, the Imperial War Service Gratuity. There's a complementary document, "Documenting Military Service in Canada and Abroad 1885 - 1918: A Short Guide to Sources" on the BIFHSGO web site.