All houses have many sources of moisture, which can damage windows, interiors, exterior paint, and siding. Protection is much better from outside moisture than inside moisture!
Sources of moisture: us humans, laundry, showers, cooking, humidifiers, gas heat or cooking. It may come from the house itself: wet plaster, or a wet or unexcavated basement.
Water vapor travels invisibly to drier areas and forms moisture or frost. It may be on inside walls, on walls or ceiling, or as iceballs in an attic right below nails. You may see water or ice on windows, moisture on the walls or floor of basements, or water blisters under the exterior paint. This blistering can extend to damage the siding, making the paint peel; or cause rot, mildew, stained walls, and sweaty windows.
Tyvek allows moisture to travel both ways. Fome-cor allows mostly one-way travel, due to separate perforations. Remodelers often find moisture and rot under Tyvek–It is not completely resistant to water. It would be little help under vinyl siding, especially in a wetter climate.