It's obviously about offence, it always is.
Coining a new word is always about offence?
In this case it seems people online discussing trans issues in the 90s wanted a quick term to be able to refer to non-trans people. If your viewpoint is that a transwoman is a woman and a transman is a man then it just makes sense to have a self-clarifying word.
Saying "Why do we need more words" just belies a lack of understanding of language in general. There's always some new specificity that our language currently doesn't cover and people will naturally fill in that gap with something that makes sense. In this case 'cis' can be thought of as the opposite of 'trans' in terms of latin prefixes so it all makes perfect sense.
You may never need a word to cover that specificity, but others clearly do or the word wouldn't have caught on.
Of course all of this assumes you're asking questions in good faith rather than just having a performative tantrum.