Quantity
When measuring things, a whole number might not be the best answer.  A distance might be longer than 5 meters, but less than 6 meters.  Meter sticks are marked off in parts of a meter.  Decimals are formed by marking off a ruler in ten equal parts.  Each part is a tenth.  5.2 meters is a little bit longer than 5 meters.  5.7 meters is a little bit shorter than 6 meters.  Each of these tenths can be marked with ten smaller parts.  The one hundred centimeters on a meter stick are done this way; by marking ten equal parts, then marking ten equal parts of each of those parts.