I feel like I'm on the verge of the same revelation but not quite getting there. What was the tipping point for you?
Given every script that inherits the interface needs to provide its own method still, it feels like there's always an alternative to using interfaces to me but I'm sure I'm not seeing the full picture. It does also feel odd to have a floating c# script that isn't attached to anything for the interface itself. I always wonder how exactly Unity/the game knows of its existence but I guess it's keeping a track of every file in the project folder(s).