Smooth’s the surface: Underlayment provides a smoother and more predictable surface for the floor covering than the subfloor, which serves a mostly structural role.
Improves adhesion:In the case ofcement board, this underlayment provides a god surface for ceramic tile to bond to. Tile does not adhere well when applied directly to a subfloor, which is known to expand and contract.
Improves structural stability:As a residual effect, a hard underlayment can give the entire floor better stability. It is particularly useful in this function in older homes, where the subfloor may be constructed with boards rather than OSB or plywood sheets.