"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"