One example is when I have an oscillation. I'd want to mark the upstrokes or downstrokes to tell them apart immediately and be sure that I'm duplicating or modifying what I'm suppposed to be, maybe easily adjust them as a group (with the fancy new Adjust tool. ; ) ).
Another is in a standard walk or run, where I'd mark the point where the foot is planted, and when the foot leaves the ground again. The timeline gets a bit messy when I have to stabilize the foot on the floor with a combination of a few extra keys and some custom curves so it doesn't smoosh into the ground or slide around.
I'd also mark the ideal entry point of the walk/run loop in the timeline— this is normally somewhere in the middle, closer to the end, when constructing and animating it in the classical way. Animating it with the entry point at 0 is really messy in comparison. (The entry point is something that's implemented on the code side. Just a state.time = entrypoint right after state.SetAnimation. I'm not asking for that to be natively supported by Spine animation data in the json though. In other entry point cases, it's something that also gets adjusted based on game balance and not something the animator should have to do.)
In this case too, there are bouncy things that lag behind the actual foot plant, like a sword in a scabbard that bounces about 1 or 2 frames after the foot plant. Meanwhile, these keys are surrounded by other keys that define other little things.
For a lot of key pose-based animations with swaps, the slot image rows get a bit difficult to read and messy to move around. Actually, this is probably a separate feature that'd be handy: If you mouse over a slot image key in the dopesheet, it'd be cool if a mouseover tip would show what that key was doing: by showing the name of the attachment it keys, or showing the image, or null, or I dunno. It'd probably also be nice if a slot image HIDE had a different color or symbol (for the same reasons).
There are a few other cases I can't recall off the top of my head. It does come up though.
