The pickelhaube was based off Russian and French helmet designs and the spike was just aesthetic. In fact, the spike made it easier for cavalrymen to knock the helmets off.

In the First World War, the Germans made helmets with detachable spikes because the spikes were easily identified across trench lines, and the pickelhaube was replaced by the stronger and more practical iconic stahlhelm.

The helmets did provide fair protection but they found these helmets were ineffective in 20th century industrial warfare and the introduction of the stahlhelm revolutionised German infantry and decreased head combat wounds dramatically.

The point of the spike wasn't defensive, but to mount a horsehair plume for formal dress, as seen here. The helmet itself was, like many uniform details in the early days of WW1, merely a product of an earlier time, and was by 1916 entirely replaced by the Stahlhelm. It was made out of leather, and provided no protection against shrapnel.

Related question - is there any factual basis to the story that pickelhaubes were removed because German soldiers were being injured by them. (i.e slipping in a muddy trench and landing on another guy's helmet) ?? Or was it just something to do with having a simpler manufacturing process for uniforms?