The first answer offers the relevant quotation from Bradbury's novel that sums up Clarisse's thoughts about porches, gardens and rocking chairs. These three objects have a number of things in common relevant to their status in this Dystopian future. Mainly: all of them encourage contemplation. It is true that all three can encourage social interaction, but sitting on porches, gardening or cooking in solitude can also stimulate the mind to creative or intense thought. In this society where books are banned as a way of keeping people docile and obedient, the act of thinking freely and creatively is forbidden. Just as reading books can unlock a person's ideas and unleash knowledge hereby encouraging dissent and initiative, contemplation arising from soothing activities such as rocking and gardening can do the same. The contemplative mind is the mind that recognizes injustice, tyranny or chaos; and the government in this society wishes to prevent its citizens from becoming too aware of the oppressive conditions under which they live.
A recent experiment conducted by NPR sought to explore recent research findings that suggest too much smartphone activity does not allow the brain to become "bored" and so discourages creative or inventive thought. In much the same vein, the citizens of Fahrenheit 451 are encouraged to interact with technology and have constant low-grade mental stimulation, but not to engage with more subtle activities that might provoke critical or inventive thinking.
“No front porches. My uncle says there used to be front porches. And people sat there sometimes at night, talking when they wanted to talk, rocking, and not talking when they didn't want to talk. Sometimes they just sat there and thought about things, turned things over. My uncle says the architects got rid of the front porches because they didn't look well. But my uncle says that was merely rationalizing it; the real reason, hidden underneath, might be they didn't want people sitting like that, doing nothing, rocking, talking; that was the wrong KIND of social life. People talked too much. And they had time to think. So they ran off with the porches.”
Clarisse describes how people used to chat on the porches or just think. Front porches, gardens, and rocking chairs are where people can gather together and socialize. They are also places where people can relax and enjoy the beauty of the outside world. By removing these things from society, people lose communication with one other. People no longer sit on the front porch chatting with one another and greeting passing neighbors. They also no longer appreciate the beauty of the world. Sitting on the front porch, you take in all the sounds of nature, like birds singing, and began to appreciate the outside world. Once that is taken away, people gravitate inside and lose that appreciation for nature.