In marketing research, why is sampling important?

Sampling is important in all kinds of scientific researches, and not just in marketing research.

Now if we want to study anything - just anything- we need a ''thing'' to study, and this ''thing'' is our sample. For instance, if we want to study the consumer behavior in the fashion industry, we will need to study a sample of consumers if we are to make any conclusion about the industry. This's something natural, but what's crucial to any research study is what SAMPLING STRATEGY should we apply.

We need to consider the followings when we conduct any study:

1-Random or Selective sampling: If you wish to make your conclusion generalizable you usually apply random sampling. If you want to study a phenomenon, you normally select a specific case to study, and your sample here consists of less units so you can study them deeply so you can draw a reliable inference (qualitativeresearches require small samples) 

2-How to select a representative sample: This is crucial if you wish to generalize your results. Perhaps the number of units is important here, but what's equally crucial here is HOW to select your sample. Sometimes you can't just randomly make your sample. For instance, let's say you want to know study what consumers think of a certain product through scrutinizing what people write online. This study will not give a valid conclusion if you survey a country with low internet penetration.