Conversion of mass can be used in everyday life if you have one type of measurement and need it in a different unit. The metric system is very easy to convert masses as each unit is based on a power of 10 conversion. An example of using mass conversion would be needing to convert grams of medicine needed into number of miligram capsules. The metric system makes it easy to convert into different units simply by moving the decimal point.
Another example of everyday use is mass producing a recipe. Say you are making an extremely large cake that is twenty times the original recipe. If the original calls for 6 cups of sugar, you might need to use mass conversion. The conversion would come if the bags of sugar were sold in pounds and you had computed the ounces of sugar.
Conversion of mass by using the U.S. Standard is much more difficult, as you must know the ratios between each unit.