In 1916, during the First World War, the Germans dropped the idea of using the pickelhaube because it wasn't practical in the trenches. I read the wiki, but I am left with one question:
It remained in service for more than 70 years, it doesn't seem to be practical at all and its additional combat value looks negligible. Did it have any real practical use?
[Source: Dr Max Litthauer’s pickelhaube (spiked helmet) with camouflage covering, 1914–1918. Donated by Bart Ullstein © Jewish Museum Berlin]
Helmet spikes and flanges were originally intended to deflect saber blows. Those on the Pickelhaube are somewhat stylized, but they still served the original purpose. Source: German Wikipedia.
I thought I remembered something similar from the English Civil War, but a bit of googling got me nothing.
The main utility of spiked helmets is against cavalry, specifically to deflect sabers being swung from "overhead."
Its value basically declined over the course of the 19th century, as the growing ease and efficacy of using rifles made cavalry obsolete (as opposed to mounted infantry). In the early 20th century, it had no utility in the trenches, where cavalry was worse than useless.
The Great War has a short segment on "the spikey helmets".
The original Pickelhaube was made out of hardened leather. As WWI progressed and Germany experienced shortages, thin steel, tin, felt, even paper was used. As you can imagine, this wasn't much use as a helmet in a modern war. They were expensive, fragile, uncomfortable, and didn't offer much protection against bullets and shrapnel.
The cover had the regiment's number on it for identification.
It was gradually replaced with the cheaper and far more practical steel Stahlelm in 1916.
Did it have any combat value? Prior to WWI shrapnel and head wounds were less of a problem, and hardened leather could turn away a saber blow. WWI brought lavish use of machine guns, snipers, and artillery and suddenly a fancy leather hat was next to useless.
The metal spike was purely for show. It was probably copied from Russian helmets and originally had a horsehair plume.
Tsarist Russian Pickelhauben, with detachable plumes, mid 19th century Source
Similar spiked helmets were used in many armies as part of full dress uniforms and less often as part of field uniforms. As Schewm's post says, the Pickelhaube was probably copied from similar Russian Spiked helmets.
Some of these images show US army spiked helmets.
https://www.google.com/search?q=uS+army+spiked+helmets&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjPhPrU4svOAhWFOiYKHVPJAREQsAQIHQ&biw=2560&bih=1826&dpr=0.51
Here is a sculpture of a US army Indian scout in full dress uniform from about 1890. Obviously Indian scouts would not wear full dress uniforms with spiked helmets while scouting or in battle.
http://www.planetfigure.com/threads/us-indian-scout-1890.60330/2
So the Pickelhaube style spiked helmets were mainly used for show in the US army and were not part of field dress for combat, even if the soldiers in the German and some other European armies with stricter uniform regulations might have worn them in combat.
The main practical use for Pickelhaube style spiked helmets was to attract voluntary enlistments as part of fancy full dress uniforms, which was more important in the British and American armies because they relied on voluntary enlistment instead of the draft up until WWI (except for 1863-1865 in the US Civil War).
Without doubt, originally to deflect sword blows to the skull. A very old practice. Finally, it became decorative with spikes being detachable for use of the helmet in combat, by which time the helmet was probably just a means of identification.
As pretty much every other response has said, they were used to deflect saber blows. Though they were useful in years prior, the onset of trench warfare rendered them useless in almost all regards. They were costly, ineffective, uncomfortable, and outclassed by the newer Stalhelm for practicality. There is rumor that the spike was used as a last resort weapon (for ramming), but this would have been very impractical and outright harmful to the wearer. While ramming, you would only see the ground, the impact would severely damage your neck, and it wasn't nearly sharp or long enough to have a effect besides -possibly- stunning the target. Even if this did happen, you would likely be in more of a daze from all the force of the blow on your neck.
