Why define the "purpose "?

Some beginners asked some questions about the test document template some time ago. One of the questions was that they could not understand the template, with the template, I still don't know how to write the document.
After thinking about it, I think that the problem is not just that you cannot understand the content in the template. The key is that you have not figured out the purpose of writing a document or doing a job. For example, to make a test plan, the final plan document is just a carrier. It is important to do the plan itself. If you first understand why you want to make a plan, what do you need to consider when you do it, why do you need to consider these things, and how to determine the things of this item, at last, I should be clear about why I should write a document to record relevant items. The same applies to other tests.
Why do you have to have a reason or a purpose to do one thing? This is to take this purpose as a benchmark and always guide you in the right direction without causing any deviation.