IIRC, it was originally made (as SR4759) for military reduced loads to mimic long range performance at short range for testing purposes. I have a jug that I purchased from GI Brass years ago.

There was a lot written about using it for cast bullets and then they quit making it - but it still works well if you have a supply.

It is also great used as the kicker under a charge of BP - one of those improperly maligned duplex loads. When used in that manner, the overall fouling is reduced and easier to manage. One grain of 4759 is roughly equivalent to four grains of BP and this can be quite important when you can't get enough modern BP in the case to allow a BP double to regulate. Currently using 8grs of 4759 under 120grs of Swiss 2fg (152gr equivalent) in my 500bpe double. Gives a 340gr paper patch bullet 1925fps... yet the two barrels are still shooting about 2" apart at 50yds. Still working on it.

Duplexing isn't allowed in a lot of competitions, so there's that.

I haven't used it a lot for cast loads in rifles as I usually use WC872 for that.