Suppose a chemist performing thin film deposition used 10 sccm of water vapor during the process. Now, he needs to calculate how many grams of water he used per minute. By definition, sccm is standard cubic centimeter per minute at standard temperature (273 K) and standard pressure (1 atm). He then calculates the mass flow rate by multiplying the volume flow rate by the density as
MFR = sccm * 18.015 gm / 22400 cm3
however, he knows that 18 gm of water at STP does not occupy that 22.4 liter volume. In another word, at STP, water does not exist in vapor/gaseous form.
So, why do we define a physical quantity in a nonphysical way?
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