- to keep the wearer's head warm
- to protect the wearer from the weather, both sun and rain.
- as part of an accepted costume or uniform.
Hats as parts of a uniform can function as part of the rank-designating features of that uniform, in that different hats may be worn by different ranks. Outside the military, a teacher's mortar board or a priest's birretta help to indicate the calling and status of the wearer.
On the other hand, the beret, boater, panama, cloche hat or school cap help distunguish schoolboys and girls from those of other schools, and other children not subject to a school's rules and discipline.
Hats have fallen out of fashion in most of Western Society, except under special circumstances. To wear a hat in these times virtually implies that the wearer is under special rules of behaviour, either being under authority or in a formal social situation.
It was, and still is, common for hats to be removed by gentlemen as a mark of respect to each other, to ladies, or to an authority figure. Consequently, the wearer of a hat may be the most senior person present. The ultimate "hat of authority" may be held to be the crown worn by Kings and Queens.