The first was that it's a lot easier to generate a few characters on a background that has nothing happening to it rather than generate a full-screen sized picture all the time. Secondly, it saves a lot of energy. Old monitors lasted much longer if they weren't asked to run a lot of bright things across them. This tied in well with the use of terms like desktop, folder, etc. Today, video technology has changed.
New monitors use less energy and produce much sharper, more colorful images.
However, that doesn't mean we're all required to stare at white screens all day. In fact, we probably shouldn't. The human eye is always drawn to the brightest thing in the field of vision. We are sensitive to too much light, which is why we wear sunglasses when it's bright out. We shade our eyes to see in the distance.