I always thought it was made of a special material, but not metal.
It would be logical to assume it was to protect the wearer from phaser and disruptor discharges, so the material has the abilities of dissipating such discharges, minimising negative effects on the wearer.
But seeing in the show a dead Cardassian with barely scorched armour and not even a hole... I guess they didn't want to damage expensive props, but the in-universe result is - they had no function.
Still, if I had to theorise, I'd say they were to protect from modern, energy weapons.