grammar - Why is redundant language bad?

While editing a document, I noticed Microsoft Word flagged "In order to...." as bad grammar. The reason being that it is redundant because just saying "To" says the same thing. This seems like weird thing to flag as "In order to do X, we are doing Y" sounds more natural than "To do X, we are doing Y", because the "In order" part seems to emphasize that X is really what we care about. (This is just an example, I'm interested in understanding the grammar rule behind this flag)

I am also used to a culture of "redundancy is good", "redundancy reduces the chance of problems", "redundancy is more reliable"; So my understanding is that redundant language should be good, as it reduces the chance that the text (and its meaning) will get misread/misinterpreted. And while I see a lot of things online saying things like "redundancy is bad" and "avoid these redundant language", I don't see anything explaining why.

So why is redundancy bad? And why should I avoid it (aside from the fact that leaving colored squiggles under things would drive me crazy)? Or in other (redundant) words, if the sentence is still valid, why does this grammar rule exist?

Answer

Redundancy is neither good or bad by itself. It is a tool, which can be used well (for emphasis or, as you wrote, for reliability) or poorly (verbosely).
I might be stating the obvious, but style is not a matter of "correct" and "wrong", like imperative grammar rules; it is rather a matter of good judgement (as well as taste).

I would go with the attic style ("characterized by purity, simplicity, and elegant wit"), keeping in mind that the aim is to make the text or speech easily understood. To avoid repeating a word, one could use personal pronouns or the like ("she", "he", "her", "him", "it", "the former", "the latter", ...), as well as synonyms. But as soon as the sentence becomes ambiguous or unwieldy, it might be better to repeat; and when the message appears too weak, it might be better to emphasize.

We all see the merit of reducing word count and we should certainly do it, when that cuts the clutter -- but never to the point of compromising clarity.

Back to your original question, banning "in order to" altogether seems a little far-fetched. Indeed the two additional words "in order" should be used for a purpose (emphasis or formality, see what Cambridge Dictionary has to say about this). So the decision would be on a case by case basis, with a preference for "to".

I would thus take this recommendation of Microsoft's spell checker for what it is: the output of a simplistic^* algorithm based on probabilities ("to" is statistically more often appropriate than "in order to").

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^* Simplistic here is not meant to disparage the technology of spell checkers; it is relative to the nuances that a human being can perceive.