To start, here is a little review about sampling in general.
The purpose of sampling is to provide various types of statistical information of a qualitative or quantitative nature about the whole by examining a few selected units. The sampling method is the scientific procedure of selecting those sampling units which would provide the required estimates with associated margins of uncertainty, arising from examining only a part and not the whole.
In any research investigation situation the advantage of sampling is as follows:
Greater economy : The total cost of a sample will be much less than that of the whole lot.
Shorter time-lag : With smaller number of observations it is possible to provide results much
faster as compared to the total number of observations.
Greater scope: Sampling has a greater scope regarding the variety of information by virtue of
its flexibility and adaptability.
This provides an explanation of alternatives to random sampling with examples.