I guess you are either talking about waterproofing membrane or soundproofing membrane.
If the installation is in a condo, a second floor area, or balcony floor, it is very important since it will not only protect your investment, but also will ensure peace between tenants of the building.
Waterproofing will enhance longevity of concrete, plywood, wood in general (when it is exposed to weather elements).
Soundproofing will reduce noise transmission through floor/ceiling between units, giving more privacy and avoiding unnecessary arguing between neighbors.