What is the importance of standard units in everyday life?

For the simple fact that it's standardized, common, and easy to use. A liter is the same everywhere and it's easily converted to smaller units like millileter because it's divisible by 10's, 100's, or 1000's, which you can do easily in your head. A Gallon, on the other hand, has different amount in England, and it has 4 quarts or 16 fluid ounces in it.