Inevitably, it's used in situations such as "as I have clearly said".
Firstly, we are none of us best-placed to judge the clarity of our own language use to others.
Secondly, if you have to tell other people you were being clear, you're being condescending. It's telling them they're too stupid or inattentive to have realized on their own how effective your communication was.
Until now, I'd never realized how insulting nominal clarity could be.
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My vocabulary bugbear comes from radio where leading questions often result in the interviewee beginning their reply with "Absolutely".
That is always what I intend to convey when I use it, personally.
Half an hour later he came back in and said, "Yes, gentlemen, it is obvious"
The half-hour detour incident in your story has never happened to me, although there have been a few occasions when I've looked at what I've just written, thought about what I've just said, or stared at my slides for a few seconds and thought "hang on... is that actually right?" before then either realising it is, or spotting the error and correcting it.
We'd done the statistics in the first four weeks and decided we were reasonably likely to do it most lessons unless he modified his behaviour.
He never did.
So why did you have to state it?
The effect is to either irritate because there was no need to explain if it was obvious, or to patronise because you're supposed to somehow have this information already and don't.