"For all intents and purposes" is just a shorter way of saying, "this isn't necessarily true, but it's close enough to being true that we may as well act like it is."Fool-proof even makes more sense...
I've always thought "intensive purposes" actually made more sense than "intents and purposes". What does it actually mean to say "For all intents and purposes, I am a good business owner" versus "I am a good business owner"? It doesn't seem to add anything. While "intensive" refers to things which are intense; it makes sense to say "for any intense purpose, I am a good business owner". And this would have a meaning like "For anything really important, I am a good business owner".