For example, a group layer dedicated to one character.
You can animate the group with first/last key tools, so you can make your whole animatic in rough first, and then later add clean layers and coloring layers in that group on top of the rough layer… so all the movement is preserved and also applied on the new (clean) layers in the group.
this way you have much more editing and drawing flexibility which you absolutely don’t have with ‘‘merging’’ layers.