Aboard smaller airplanes it becomes more significant. There are always some passengers who carry heavier baggage than normal, and there are always some passengers who pack "light."
With large passenger loads, such as 400 pax in a B747, these differences average out and the "standard" pax weights and "standard" baggage weights applied seasonally [heavier in winter] are surprisingly realistic.
In this age of globalization, where east meets west, the passenger mix of asians [who weigh less than industry applied averages] and westerners further "compensates" for the excesses.
2000kg overload in a B747 is insignificant; 200kg overload in a Cessna 172 could be catastrophic. Therefore, on very small airplanes [less than 50 seats] it would be well to do to check individual passenger and baggage weights.