To hold the pickerel rig on the bottom of the river when it's flowing fast. The river is more silt than rocks so the weights need to be beefy sometimes or the current will just sweep it away. I normally use just a little egg shaped weight or that long skinny one.

I've tried using them as a slip weight too but I don't carry floating lures or floats so that's less practical unless the river is just right.

The pyramid shaped ones act more like an anchor than the round ones do but they snag on stuff more easily too.