During WWI, the German army issued about a half million sets of trench armor, often called grabenpanzer or sappenpanzer. Despite common belief, this armor was not intended for trench raiding – in fact, German orders regarding it specifically prohibited this use because of the amount of mobility lost when wearing it. Instead, the armor was for sentries and machine gunners. These were soldiers who tended to be stationary and easily targeted, and the armor was intended to help protect them.
American testing after the war suggested that it could stop a rifle bullet at 400-500 yards. That would not be very useful in a trench raiding scenario, but for someone like an MG08 gunner under enemy machine gun fire from a distance, that could be enough protection to stop otherwise fatal hits.
This set at Rock Island includes a set of the torso armor, plus a Stahlhelm, reproduction liner, and armored browplate as well.
Since filming this video, I have talked to International Military Antiques and gotten a reproduction set of the armor from them, which I plan to do ballistic testing on next week. Should be interesting to try it out firsthand!