These badges were produced to denote that a civilian during the war was engaged in important war work. Many were produced privately by companies for their employees.
Conscription was not introduced until 1916 and up to that point it was commonly assumed that a man not in uniform was a shirker or coward. Some of these men were given a white feather.
These badges were therefore produced to prove that the wearer was serving their country in a different way; just because no uniform was evident they could be working in munitions or dockyards, both vital to the war effort.
After conscription was introduced in 1916, the need faded but they were still used especially by female shift workers, the wearer getting priority boarding and fare concessions on public transport, as well as stating that nothing was disreputable about these ladies travelling alone at night.