The same reason metres are used to measure houses instead of miles. One is of the right size at that scale, the other is so large all measurements would be in very small decimals.
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The reason that you can be more precise while measuring in millimeters is because millimeters are smaller than centimeters, therefore you can make more specific measurements. +++ 1m = 100cm and 1cm = 10mm. The cm is not a 'preferred unit' in SI scales.
An inch is much bigger then a millimeter. There are 2.54 centimeters in an inch, so there are 25.4 millimeters in an inch.
To convert millimeters to centimeters, you can use the fact that 1 centimeter equals 10 millimeters. Therefore, 5 millimeters is equal to 0.5 centimeters. Since 8 centimeters is much larger than 0.5 centimeters, there are no 8 cm in 5 mm; rather, 5 mm is only half a centimeter.
because most objects viewed are so small that millimeters are to big to measure with.
1) It is an international standard, so all scientists use the same measurements. For comparison, for example, different countries have different definitions of a pound. 2) Calculations are easier, because of the decimal prefixes.