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The metre is the fundamental unit, but they are all metric units.
There is no metric unit for volume which is equal to 1.057. All measures of volume MUST have measurement units: these may be cubic to start with (gallon, pint, bushel) or derived from linear units (cubic inch, cubic yard). 1.057 is a pure number which has no units. It cannot, therefore, be a measure of volume.
All of them. Whatever units a country uses are 'customary' in that country.
Mainly to use an accepted standard (scientists all over the world use the same units). Also, it is easier to do calculations in metric units.
The Mint switched to metric units for all coin specifications several years ago. See the Related Link for a list of current coins.