From Porch to Patio, a 1975 piece by Richard Thomas, discusses the transition in American society from the semi-public gathering place in front of a house to the private space in the back.
When a family member was on the porch it was possible to invite the passerby to stop and come onto the porch for extended conversation. The person on the porch was very much in control of this interaction, as the porch was seen as an extension of the living quarters of the family. Often, a hedge or fence separated the porch from the street or board sidewalk, providing a physical barrier for privacy, yet low enough to permit conversation.
When people started moving out to new buildings in the suburbs, the patio emerged to provide the privacy for these urban refugees.
The patio was an extension of the house, but far less public than the porch. It was easy to greet a stranger from the porch but exceedingly difficult to do so from the backyard patio. While the porch was designed in an era of slow movement, the patio is part of a world which places a premium on speed and ease of access. The father of a nineteenth-century family might stop on the porch on his way into the house, but the suburban man wishes to enter the house as rapidly as possible to accept the shelter that the house provides from the mass of people he may deal with all day.
(via front porch republic)