We often talk (OK, a lot) about getting your flies down, down, down into the fishy zone. So often you're putting in the perfect cast with the perfect drift only for the pointy end to zoom overhead sight unseen.
In the past if your flies onhand don't have enough weight already built into them or you'd like to drift something past without a bead on it you've usually had to incorporate split shot which can be a tricky affair to get right, not to mention the damage done to the leader by crimping them down.
Check out the better alternative to getting some weight into your rig to get your flies in the dangerzone, the Loon Deep Soft Weight.
"This charcoal-colored nontoxic, Tungsten-based product is biodegradable and reusable, making it an environmentally friendly alternative to split shot and other lead based weight. The weight and depth are easily adjustable, to make sure that a nymph always finds the bottom. Pinch off desired amount from container. Apply to leader in the shape of a football at desired location. Dip in water to harden. Remove or change the size when needed."