When researching the development of a phrase it is usually the origin that is difficult to determine; the spelling and the meaning are generally pretty well established. With 'for (or to) all intents and purposes' it is the other way about. The origin is unambiguous, as the first recorded use was in an Act of Parliament under Henry VIII, in 1546:"to all intents, constructions, and purposes"Henry didn't shilly-shally when it came to the law of England. In 1539, he had Parliament pass the Statute of Proclamations, which gave him power to legislate by proclamation. In effect, from that date, the law was 'to all intents and purposes' whatever Henry VIII said it was. He made good use of this freedom; as well as executing a brace of wives, many others with whom Henry didn't see eye to eye were hanged, beheaded, burned or boiled, often for quite minor offences.
The meaning of 'to all intents and purposes' is less clear. 'Intents' and 'purposes' are words that we aren't likely to come across very often in the 21st century, outside of legal documents. The phrase, like many a legal phrase before and since, is rather obtuse and difficult to decipher. What is meant when a thing is said to be true 'for all intents and purposes' is 'it isn't actually true but it is so close to being so that we may proceed as though it is'. An example may help:Isaac Newton's Laws of Motion aren't 100% precise in describing the motion of objects that approach the speed of light. However, for a man on a horse who measures time by a pocket watch, they are, for all intents and purposes, accurate.Nor is the wording universally accepted. As is seen above, the original form is 'to all intents and purposes' but it is 'for all intents and purposes' that is now more widely used. No problem with that. The real debate arises with the many examples in print of 'for all intensive purposes'. Mistakes of this sort often date from the Internet era but this one was current as early as the late 19th century. An example comes from the Indiana newspaper The Fort Wayne Daily Gazette, May 1870:He has never had a representative in Congress nor in the State Legislature nor in any municipal office, and to all intensive purposes, POLITICALLY SPEAKING, HE MIGHT HAVE WELL HAVE BEEN DEAD.The recording in print of the 'intensive' version, written by a professional author, suggests that it may have been in public parlance for some time before that. It is an understandable mishearing, as the original is hardly intuitive and the alternative makes a sort of sense. It is, however, an error. We should think ourselves lucky that King Henry is no longer in charge of the punishment of minor misdemeanors.