The built-in lens "all-in-one" designed to be digital, which means it is specifically matched to the CCD. Cameras offering interchangeable lenses are designed for 35mm film, which usually does not have sufficient resolution for a Megapixel CCD sensor. When interchangeable lenses are removed from the body, the CCD and internal mechanisms are exposed to dust in the environment. Contaminants are a much bigger problem on digital cameras since the CCD is much smaller that a 35mm film plane.
One of the primary reasons for using interchangeable lenses is for a wide variety of focal lengths. The E-20N contains a 4x optical zoom 9mm-36mm (35mm-140mm equivalent in 35mm photography) built-in lens. In addition, there are wide angle, telephoto and macro add-on lenses. Some interchangeable lenses cost as much as the E-20 camera itself. Other lenses that were originally designed for film cameras do not deliver same focal length on a film camera as a digital camera. For example, a wide-angle 28mm lens attached to a digital camera would only provide 56mm equivalence.
This system offers great flexibility and will suit approximately 95% of most shooting needs.