In what circumstances should it be preferable to use for when it has the same meaning as because?
From Grammar Girl:
I was tired after my journey, for I had been forced to bike 20 miles.
However using for at the beginning of a sentence is considered bad/improper by some. To find some more specific information, please read the first link, it explains usage of for quite well. Also when for us used as a preposition, it is different.
For instance, this sentence uses for properly.
"for" in function of "because" is a bit of antiquated though still used and rather elegant construct. You won't find it in slang often, and neither does it appear in technical-formal language, but definitely appears in formal speeches, in high society, poetry etc.
"I choose miss Ada for she has the softest voice"
Note this does not apply to replacing "because of", where "for" is just as informal:
"I choose a cheesburger for its yummy meat".
There are a couple major differences between "for" and "because."
-As conjunctions, either works, but "for" sounds slightly formal and archaic. "I am late because the roads were very snowy this morning." and "I am late, for the roads were very snowy this morning." both work, but "because" is more commonly used.
-"For" is used as a preposition, but "because" isn't. You could say, "I stopped for lunch," but not "I stopped because lunch." In some cases, you can use "because of" but it often has a slightly different meaning than "for." (For example, if you say "I stopped because of lunch," that only tells me that lunch caused you to stop whatever you were doing. It might be that you stopped to get lunch or that you stopped doing something because other people getting lunch was interfering with it. If you say "I stopped for lunch," it's clear that you stopped to get lunch.)