Back in the days of imperial measurement, 1/16 inch was (and still is) a pretty universal standard sheet metal thickness, and the original standard for PCBs.
And 1.6mm has now become the new metric standard for many things, as its much closer to the old 1/16 inch than 1.5mm.
1.5mm = .0591"
1/16" = .0625"
1.6mm = .0630" its only half a thou different.
And that may be important where boards plug directly into card cage runners and edge connectors which were once very popular.