It was ornamental. The helmet design relates to old school Prussian pride in arms, excellence in drill and impressiveness of appearance. The spike detached and often held a plume. In world war one it ended up being phased out and replaced with the stahlhelm, the familiar "coal scuttle" helmet the nazis wore in the second world war. You have to remember though that at the start of world war one, most nations went to war in formal dress (the french for example went into the western front wearing bright blue jackets, red trousers, white gloves and cloth caps and left it wearing drab and muted blues, greens and wearing steel helmets) the kaiser famously said that his troops would be home before the autumn leaves fell but when the war turned into a stalemate, the only protection option they had was their boiled leather, spiked, parade ground formal headgear. Its use though was merely for show, similar to a shako cap. For more detail on the helmet design and adoption in the mid 19th cent. Try visiting this site: http://www.worldwar1.com/sfgph.htm