That's a very beautiful and elaborate example of an armet, a type of highly fitted helmet. It was probably the pride and joy of one particular person, who would always wear it any time a helmet was called for. If you captured such a helmet it'd be unlikely to fit you, so you'd hang it on the wall for a trophy, or ransom it back to the owner.
Armets evolved into the simpler hounskull (also called a "pig face" or "dog face") bascinet. Bascinets do not have the hinged and locking cheekplates or the extremely dished out back, so they can fit a wider range of head shapes.