Some underlayments come with a pre-attached moisture barrier. You might be wondering why would I need a moisture barrier? Well with wooden sub-floors the moisture barrier is meant to protect your sub-floor from spills. The moisture barrier prevents liquid from getting to and swelling your wooden sub-floor.
While dealing with concrete sub-floors, the goal of the moisture barrier is to prevent moisture from making contact with the bottom side of your flooring. If you have moisture sensitive flooring with no moisture barrier on a concrete sub-floor, your floors will be exposed to moisture from the concrete and will swell and even possibly start to mold. If you install floors resilient to moisture so you don’t need a moisture barrier, it is still best to have one. The moisture will affect the air in your basement, making the humidity higher.