I’ve seen this abomination of a bastardized phrase in student writing for years, but last week I actually caught it in an email from a fellow educator. Oh, the shame!
The correct phrase, of course, is “for all intents and purposes.” For all intents, and for all purposes. It’s a legalistic cliché that claims that a given idea is true in every practical situation, even if not technically accurate (de facto, as opposed to de jure, or connotation rather than denotation).
Example: “For all intents and purposes, the boy she’s taken care of all his life is her son.” The hypothetical boy in question isn’t the woman’s biological offspring, but her motherly care while actually raising him means that he might as well be.
The incorrect version would have to mean something like, “For all purposes which are really serious,” perhaps. I’m not sure what people have in mind when they try it.
Why the confusion in the phrasing? Just say it out loud. It sounds like that; the word intensive is far more common than intents and.
Which means that people know the phrase because they’ve heard it, not because they’ve read it. So this is yet another example of our increasingly illiterate society, driven by mere oral and visual communication, mangling something that can only be understood fully in print.