I think having an inner and outer shell (sandwich construction) can have practical benefits:
- Reduces friction between layers (Nano Air is especially good in this regard)
- Reduces shedding of micro plastics
- Can improve durability, if the outer layer is made from a tougher material (e.g., Proton series)
- Can improve next to skin feel, when worn directly on skin (e.g., Nano Air)
- Generally more compressible, although sandwich construction usually applied to thicker, higher loft articles of clothing and pile is generally used for thinner, low loft articles of clothing.
I have both pile and sandwich midlayers and I wouldn’t say that either type is always superior to the other.