Visualize the post-war era between the 40's and 60's when most Mid-Century experts indicate the beginning of this movement. That period is when the Boomers' lifestyle created a demand for suburban modern tract homes as they established their nuclear families.
The new post-war architecture brought about uncluttered geometric lines that emphasized simplicity and utility due to limited resources. The idea was to integrate an open space setting by using natural lighting (e.g. windows around the house) and an easy indoor/outdoor transition that's exemplified in Eichler-style homes. These distinct architectural elements inspired an innovative furniture style that's called Mid-Century (Mid-Mod, Modernism, Post-Modern).