"For" in this context serves to replace "because" or "since" in its usage. It is not technically grammatically correct, as this should start a dependent clause, and as you note in the OP, the main clause is missing.
Some writers, however, will use this style to emphasize the point, despite its informality. For example, one typical use for this might be in a narrated story where the author has just ended a sentence, but then desires to reinforce the point with an additional statement, connecting to the prior sentence via this subordinating conjunction. This adds a familiar tone to the writing style.