It increases the saturation and especially the red end of the spectrum. Needs good light to work well, the magic hours are even more magic.
Not cheap though for a few frames. back in the days when stock was worth something and it hadn't been diluted by loads and loads of the same shots everywhere, then it was worth having to give your images an edge - the fact that the filter was over £600 was irrelevent. That was paid back in one shot - in those days it was fairly common to get £500 or £600 for a one use from stock for fairly ordinary images. It would take a lot of sales to claw back the investment nowadays from stock.