Why Share Your Google+ Posts “Privately”?

I just found a great and very useful post of one of my favored peers on Google+ and clicked on the SHARE icon – which is a right-turning arrow on Google+, when a window came up:
“This post was originally shared privately. Be thoughtful about who you re-share it with. You can’t reshare it publicly.”
Well, I should have seen it, before hitting the sharing button: under her name appeared “Shared privately”.
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Going back to her timeline, I found this “Shared privately” on all her articles she p...