It's all in the water, and the method of presentation. Faster pocket water requires a shorter leader to get the presentation down immediately, so it isn't out of the drift before your bait gets near the bottom.

If you're side drifting you want a long (4-6ft) leader so your bait floats along with less influence from your lead touching bottom.

Another time you'll see short leaders in use is chum fishing at river and creek mouths. This is so that your offering floats up into the fish zone when your weight is just resting on the bottom.

I'll use leaders anywhere from 18" to 6 feet in length, depending on the water conditions and what tactics I'm using at a given time. Too broad a question to answer without writing an essay.

Fish on.........