1. When fishing in deeper water (+10ft) in fresh waters
2. Using a floating fly on a sinking line for LMB. The line drags the fly top the bottom as you strip, and rises when you stop.
3. In an area that has a tidal current. The sinking line can get the fly to the fish.
4. In the surf a floating line is a tangle waiting to happen. An intermediate works, but the large fish seem to be deeper, so I like to use a sinking line.
5. Brushpiles. I'll throw a bendback on a sinking line into a brush pile and drag the fly over every tree limb. Heavier tippet is required.
6. Trolling. In know, you don't troll with a fly rod, but when you are moving in a kayak from spot to spot, the sinking line drug behind seems to get more hook-ups.