No First World War helmet was going to stop a direct bullet or shell fragment strike, making a helmet that could do so would have been highly inefficient and ridiculously heavy. What helmets could and did do was reduce the impact of ricochets and debris thrown into the air by explosions. In particular though, the design of the helmet was intended to protect the head and neck from shrapnel from shells exploding above the trenches.

Now, I don't know if you know but shrapnel is often mistaken for shell fragments, that is, the jagged pieces of metal ejected from shell casings when the charge inside explodes. Shrapnel on the other hand is the term given to the steel or lead balls encased within a shrapnel shell. These shells are detonated above head height and eject the payload of shrapnel in a cone shape down towards the ground. These are particularly destructive against groups and infantry as the shrapnel tears through flesh. The Type B 'Brodie' Helmet was specifically engineered with thicker steel across the dome to protect against airburst shrapnel and it did this quite effectively.

Ironically, after the introduction of the steel helmet, the number of recorded head wounds rose somewhat dramatically. This wasn't because soldiers thought they were immune to gunfire but rather because non-lethal woulds were recorded by type whilst deaths incurred by injuries to the head were simply listed as dead. After the introduction of the steel helmet, head injuries that would previously have been fatal were reduced and more men were surviving previously fatal head injuries. Thus, the number of head wounds increased whilst the number of lethal head wounds decreased.