There are 4 different goals of writing, depending on the writer and assignment in question.
Writers try to
3) move, or
4) administer others.
And these goals give rise to the following 4 different types of writing:
1) Copy writing (sell)
2) Technical writing (teach)
3) Fiction/Creative writing (move)
4) Business writing (administer).
Copy writing tries to sell products and services, as the advertisement-legend David Ogilvie always (and rightfully) maintained. And I agree with him. Until and unless someone buys something, copy writing is not good for anything.
Technical writing, on the other hand, is an answer to the basic question of HOW.
But in order for technical writing to explain the "how" of anything, there first has to be a clear definition of its "WHAT" and "WHY". Without that pre-requisite step, your technical writing can easily get lost in the woods.
This is usually expressed as the principle of knowing who "the audience" is.
This is just another way of saying that unless a technical writer is sure of the PURPOSE behind the document, that is, the manner in which the users will use the document to PERFORM certain tasks, then just explaining how something is done, may or may not get the job done.
Because there is always more than one way to describe the HOW of anything.
For example, imagine you are asked to write a technical guide on the HOW of running; a "User's Guide to Running."
If as a technical writer you immediately pull up your sleeves and start writing i, you might be in for a big trouble.
Let's say you devote chapters on strength and speed training, without ever thinking WHAT this guide is supposed to achieve, for WHAT kind of an audience.
What if it turns out that you were actually asked to write this guide for runners over 50 years old, who could care less about "strength" and "speed" but are keen about "losing weight"? That of course would require a totally different sort of technical guide, wouldn't it?
Copy writing excels as long as people buy the product; period.
Technical writing, on the other hand, excels only when the description of HOW matches the PERFORMANCE GOAL of the project's AUDIENCE (or, the project's "WHAT" and "WHY").