Before we answer the question let us first define what Sampling is. In simple terms sampling is taking a small portion (sample) of a population and analyzing in an objective manner such that it provides an accurate representation of an entire population.
HOW IS SAMPLING USEFUL TO STATISTICIANS?
In stats it is difficult and sometimes impossible to find the statistics for an entire population, hence sampling is a great way to gather information about a population. Let's illustrate this by the use of an example:
If we were to determine how good is a loaf of bread is, how would we approach this? Certainly we will not eat the entire loaf of bread. Rather we would eat a slice and the study and analysis of the slice will suffice as an accurate representation of the entire loaf of bread (sample = slice, loaf of bread = population)
HOW IS SAMPLING USEFUL FOR ENVIRONMENTAL ENGINEERS
In Environmental Engineering, samples of waste water, drinking water, soil and hazardous waste is generally analyzed. Let's look at an example of a dam being contaminated with hazardous waste.
Imagine an important dam in your country is polluted with hazardous waste from a nearby mine causing algae to accumulate and fish in the water to die. Are you going to analyse each fish in the dam? Or empty the entire dam? Obviously not! It will be time consuming and nearly impossible. Basically samples of water from different areas within the dam will be taken. Additionally fish from different parts of the dam can be taken for analyse. Hence, the samples of water and fish becomes a representation of the total population of the quality of water and fish in the dam.
NB: It is important to take the correct and accurate samples using the correct techniques found in statistics. If one does not use the correct technique, your sample will have an inaccurate representation of the population.