Samples are often used over population because they are more convenient and practical (sometimes it's impossible to survey your target population). Large sample sizes increase the likelihood that your findings will be representative of the population.
Basically it is because it is simpler and also more accurate. The process of survey sampling illustrates the process of choosing a sample of different factors from a target population so as to conduct a survey in statistics. Find more information here http://en.wikipedia.org/wiki/Survey_sampling.