In a poll, why must there be a clear and accurate link between sample and population?

The sample must represent the population in order to give good, clean and correct results of the poll.

If the sample doesn't represent all the aspects and ideas of the population, the results will be different from the general opinion of the population, and therefore false.

This will cause the incorrect conclusion that can late effect the different researches and their results, but also various different processes, actions and reactions of the market, campaigns and other interactions with the population.