The phrase for all intensive purposes is an eggcorn, and it makes no logical sense. The original phrase is for all intents and purposes. It means in every practical sense.
Examples
Although for all intensive purposes makes no sense, it appears surprisingly often—for example:
It was a questionable call in the 81st minute that for all intensive purposes decided the outcome. [Bleacher Report]
The student was sharing a story with another student about “Amanda” stealing her fake I.D. Slade reassured the audience that he changed the name for all intensive purposes. [The Quinnipiac Chronicle]
And here are a few examples of for all intents and purposes used well:
Northwest Europe is, for all intents and purposes, every bit as rich as America. [Economist]
For all intents and purposes, my opinion most always mirrors that of the average Canadian … [Telegraph Journal]
For all intents and purposes, the Nexus One is a T-Mobile phone. [Polyclef Software]
Even when used correctly, for all intents and purposes generally serves no purpose. Each of the above sentences would lose no meaning if the phrase were dropped.
