Normative Signs: The Poetry of "Ought"

Here’s another one I find odd. It’s posted — and it tells you it is posted — on the lawn of a private residence in South Carolina.
It’s not immediately obvious what the word “Posted” means, here, or what it adds. Sure, it’s true that this sign was posted, but isn’t that obvious? (What’s the alternative? That the sign popped randomly into existence?) What else does that word add? Is it for emphasis? Or is it merely an introduction of sorts, like the word “NOTICE” at the top of a bulletin?

I looked around online for an answer, but found nothing authoritative. Some people suggest that “Posted” is a legal formality, or even a requirement. If (e.g.,) a charge of trespassing requires that a No Trespassing sign be posted, then the word “Posted” on the sign, I suppose, makes it abundantly clear that suitable notice has been given.

I’ve seen several signs like this around South Carolina, and also in Florida. Never in Canada.
Odd also that on such signs the word NOTICE is typically in a larger font than the rest of the words. The net effect is actually that all you can see, from a distance, is that something (some injunction, presumably) has been posted. In the example just below, you actually have to get close enough to the sign that you are in violation of the sign’s injunction in order to read what the injunction is.