A sample that accurately reflects the characteristics of the population as a whole
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Because without representative sample, your results will not be valid.
Take a simple random sample.
A representative sample is one where the statistics of the sample are the same as the statistics for the parent population.
Well, if your sample does not represent the larger sample, you'll certainly not get a valid result ... For example, if you're studying pregnancy and your sample includes men - The whole idea of "representative' sample is fuzzy and often gives interesting errors.
It is important for a sample to be representative, so that the whole population's view is counted, and not biased by one particular group within the population.