Henry and one or two others have summed it up fairly well. The "were" used in place of "was" in some English conditionals is a relic of the Old English subjunctive. It preserves no semantic distinction in Modern English and can be dropped. It is still quite frequently used in a number of constructions, though, and has more or less taken on the status of a fixed expression or idiom. The notion that it is not used as often as it used to be isn't supported by a quick check of corpora I have access to.