Originally used in common American/English
vernacular as 'all intents and purposes' by just barely
unintelligent people trying to sound as if the 'intents and purposes' surrounding them are highly important.
This phrase has since been changed to 'all intensive purposes' by people who do not listen carefully to
the original phrase used in poorly written movies and who have also never seen it in print in equally poorly written books.
Also, the person listening to a speaker who says 'all intensive purposes' is likely to overlook it.