IEC 61439-2 - Form of Separation - Metallic Versus Non Metallic Barrier

sorry for the dumb question: The standard states that separation may be achieved by means of partitions or barriers (metallic or non-metallic), insulation of live parts or by the integral housing of a device itself (e.g. MCCB).

Then everyone may prefer to use non metallic (acrylic sheets); Why do metallic partitions/barriers are used then? if metallic is must, for which form it should be used?