I find that layer cake is an interesting way to introduce another axis in spatial representation. It provides a 3D visualization in specific groupings, with each “layer” representing a specific period of time. However, while the layering is pleasing to look at, I do not think it is strictly necessary. It seems to me that the layers could be removed, and time could still be represented as a continuous range on the vertical axis. That is, rather than segmenting time, time can be represented linearly according to depth. I believe doing so would be clearer and still make just as much sense visually.