Learning about any subject requires you to learn about the terminologies used in that subject. Sampling also has some technical terminologies that you should understand before you can start understanding the various sampling techniques and their advantages.
Suppose you have to run a survey about the coffee drinking habits of high school students of USA. The population of the students is about 4 million. You can not even imagine running the survey by asking each and every student to get the relevant data because of requirement of huge amount of time, money and other resources. The cost of the survey in this case would be too monumental to justify the effort. To solve these types of problem, sampling can be used.