> This is the opposite of what we've been taught at school
Not exactly. Schools don't tend to teach this concept, but there is a difference between
• regular "words" — which are only defined descriptively by anthropological observation of usage; whose usage and shades of meaning evolve memetically; which can have different shades of meaning to different people; where people using a word in a novel way can't really be said to be using it "wrong", only "unclearly" — and not even that, if their usage later catches on;
• and jargon terms — which are defined prescriptively in some originating document somewhere; where jargon can be used "correctly" or "incorrectly"; where everybody attempting to "properly" use a given jargon term is trying to convey exactly the same meaning from the originating document's prescriptive definition.
Jargon terms aren't specific to technical writing; they're quite common in everyday prose as well. For example, units of measure are jargon terms. You would never even think to try coming up with a synonym for "degrees Celcius" or "US Dollars" if you wrote it several times. You'd at most abbreviate these, or leave them off of successive entries in a list of same-united things.
The key realization of technical writing is that you yourself can create jargon terms — define them just for the purposes of your document; and that it's extremely helpful to readers' understanding of an opaque subject when you do so, because you're making it clear by doing so that this is (at least in your own presented mental model) a distinct important concept to be lifted out and thought about on its own, a proposed new mental tool in the reader's toolbox for dealing with the problem domain.