The "Surface layer 01" is just providing color that is interpreted as displacement by the "Snow" layer. In this case it appears to be a fairly basic single color since Fractal Breakup doesn't seem to be enabled. The Displacement input takes a color input and interprets that as a sort of "height" value - i.e. displacement - where white/light = higher and black/dark = lower. The Snow layer then has "Smoothing" turned on, which is an effect only the Surface Layer currently provides, so that's why a Surface Layer is necessary in that case.
- Oshyan