In Bradbury's dystopian world, the people in charge want everyone else to stay in their homes and watch TV. Porches no longer exist because they don't want people outside, where they could look around their neighborhoods and engage the people they live near.
Clarisse tells Montag that her uncle was once arrested for being a pedestrian. Why would this be illegal? If a person happened to look outside and see someone walking around, he or she may be encouraged to do the same. Walkers get the chance to explore their environments and meet each other. This would lead people to start to question things . . . which would then lead to discovery and the development of critical thinking (and intellect!) — exactly the things that the powers-that-be want to avoid.
