This ‘Cap a pied’ helm was light head armor that would have been used by a basic foot soldier, like a yeoman trooper, or an archer, usually in a militia, as protection against swords. They would have worn it over a knit cap and under a leather coverup for more complete protection. The English had more of these sorts of things in use than most other places.
Technically, the ‘Cap a pie’ or ‘de pied en cap’ means “armed from head to foot” in French. More typically, it refers to the full body plate armor periodically popular from the 1500s to the 1900s.
My version of the helm is more what a poor foot soldier who couldn’t afford the good – and very heavy – plate armor, might have worn for protection.
Either way, it’s head armor that’s intended to protect a soldier from death by violent slash to the head by very sharp metal object.