Question About Using Layer.mergeAll on a Single Layer

Hi. I was reading this code : https://effect-atom.kitlangton.com/#todos-ii
About this line

const todoRuntime = Atom.runtime(Layer.mergeAll(TodoService.Default))

const todoRuntime = Atom.runtime(Layer.mergeAll(TodoService.Default))

I have a question. What's the point of doing a Layer.mergeAll on a single layer ?

Question About Using Layer.mergeAll on a Single Layer - Effect Community