Most common use of the phrase is

"KNOW ALL MEN BY THESE PRESENT"

It is often used and sometimes requred legal mumbo jumbo at the start of a public proclamation...

It sort of like saying. "by posting this properly in a public place in accordance with the law it is safe for me to assume that everyone in the whole world know the following"

It kind of gives you the right to say "I Told you so" without really having to tell everyone.

You are making the document a matter of Public record which any truely interested party could have access to and take action with it.

It is used often on the following types of documents:

Power of Attorny where a person with the proper papers and signatures present can sign and speak for another person.

A Last Will and Testiment
Again which gives an executor the power to speak and act for the person who has just died.

A Deed for a Pacel of Land
This document again tell anyone who can read it and understand it that the Seller has SAID that the Land Now belongs to someone else and that they have permission to do with it as they please (except for certain considerations which may be preexisting covenants such as type of use restrictions. Such as saying that a town can have a parcel of land that I am giving to them as long as they build a library on the land and place books in it that are made of paper. (I personally know of one such deal where the donated library was not allowed to contain computers instead of books).

It is used in several forms as below:
"KNOW ALL MEN BY THESE PRESENT"
"KNOWN ALL MEN BY THESE PRESENT"
"KNOWN ALL MEN BY THIS PRESENT"
"KNOW ALL MEN BY THIS PRESENT"
"MAKE IT KNOWN THAT"
"LET IT BE KNOWN THAT"

https://answers.yahoo.com/question/index?qid=1005123000501

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Note added at 33 mins (2015-02-06 22:09:33 GMT)
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Hallo Rozemarijn,

Dat snap ik, het Nederlandse equivalent laat ik aan de juridische vertalers over, mochten ze hun gezicht willen laten zien.

Het leek me echter dat de persoon die in dit antwoord aan het woord was enigszins geïnformeerd was.

Voor mij persoonlijk is 'let it be known that' het meest toegankelijk.

Ik ben zelf medisch vertaler, gediplomeerd plus drs. in de geneeskunde.

Ik weet vanuit mijn gebied hoe belangrijk kennis van zaken is.
Dit geldt misschien nog sterker op juridisch gebied.

Hmm, see e.g.:

http://www.adamsdrafting.com/know-all-men-by-these-presents/
http://www.proz.com/kudoz/english_to_german/law_contracts/36...
https://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archi...
http://www.translatorscafe.com/tcterms/en-GB/thQuestion.aspx...
http://forum.wordreference.com/showthread.php?t=359107

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Note added at 15 mins (2015-02-06 21:50:59 GMT)
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HOME > GARNER'S DICTIONARY OF LEGAL USAGE > KNOW ALL MEN BY THESE PRESENTS

"know all men by these presents"

(= take notice) is a flotsam phrase—as sexist as it is inscrutable to most readers—that needlessly begins many legal documents. Following is a typical beginning of a bond:

“Know all men by these presents that we, all of the undersigned owners of all of the property described in the surveyor's certificate hereon and shown on this map, have caused the same to be subdivided into lots, blocks, streets and easements and do hereby dedicate the streets and other public areas as indicated hereon for perpetual use of the public.”

Oak Lane Homeowners Ass'n v. Griffin, 219 P.3d 64, 66 (Utah Ct. App. 2009)

(from: Garner’s Dictionary of Legal Usage, Bryan A. Garner; online version in Oxforddictionaries.com)
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Note added at 28 mins (2015-02-06 22:03:43 GMT)
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See also:

"presents": The present instrument. The phrase these presents is used in any legal document to designate the instrument in which the phrase itself occurs.

PRESENTS. This word signifies the writing then actually made and spoken of; as, these presents; know all men by these presents, to all to whom these presents shall come. (http://legal-dictionary.thefreedictionary.com/Presents )
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Note added at 1 hr (2015-02-06 23:17:46 GMT)
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Another person I trust in all matters legal (and the author of the excellent "A Manual of Style for Contract Drafting") has this to say:

"Sometimes you just have to laugh. Or cry.

In my current guise as soon-to-be-globetrotting contract nerd, I was looking at a contract from the Philippines. The first thing I saw below the title and above the introductory clause was this:

KNOW ALL MEN BY THESE PRESENTS:

“WTF!,” says I.

…

KNOW ALL MEN BY THESE PRESENTS is beyond fatuous. Aside from its oddball components—all caps, “men,” “presents”— it’s entirely pointless. Anyone who uses it is proclaiming, loud and clear, “I’m an unregenerate dinosaur! Stop me before I draft again!”" (http://www.adamsdrafting.com/know-all-men-by-these-presents/ )
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Note added at 1 hr (2015-02-06 23:34:14 GMT)
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Hilarity on Ken Adams's blog:

PhilStudent: Fatuous, yes, I agree with you; but it's the norm here.

Ken Adams: I don't know where "here" is, but wherever it is, it doesn't matter that this misbegotten phrase is the norm. Any drafter is free not to use it; anyone who complains that it has been omitted would have to be a truly pathetic creature. (http://www.adamsdrafting.com/know-all-men-by-these-presents/ )
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Presents? Thank you very — oh.

Every time I see the silly phrase “Know all men by these presents,” I think of Christmas. Perhaps a statement the Magi wanted to make about their presents for the Christ child. Nevertheless, I’m no expert on drafting contracts: on that subject, I defer to Ken Adams, who riffs on the silly phrase in this post.

Can I get a witnesseth? (http://raymondpward.typepad.com/newlegalwriter/2012/01/prese... )
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Note added at 13 hrs (2015-02-07 11:34:48 GMT)
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Black’s Law 9th Edition:

presents, n. pI. (14c) Archaic. The instrument under consideration.• This is usu. part of the phrase these presents, which is part of the longer phrase know all men by these presents (itself a loan translation from the Latin noverint universi per praesentes). See KNOW ALL MEN BY THESE PRESENTS.

know all men by these presents. (16c) Take note. • This archaic form of address - a loan translation of the Latin noverint universi per praesentes ~ was traditionally used to begin certain legal documents such as bonds and powers of attorney, but in modern drafting style the phrase is generally considered deadwood. See NOVERINT UNIVERSI PER PRAESENTES. Cf. PATEAT UNIVERSIS PER PRAESENTES.