* To disguise Ponyboy: After Johnny kills Bob Sheldon, he and Ponyboy flee to the countryside. Johnny, who is aware of the police looking for them, wants to make Ponyboy less recognizable. Cutting his hair, which is a distinctive feature, is a way to help Ponyboy blend in and avoid detection.
* To symbolize a new start: Cutting Ponyboy's hair can also be seen as a symbolic act. It represents a drastic change, a shedding of the old identity and a stepping into a new, uncertain world. They are on the run, their lives forever changed, and this physical alteration reflects that change.
It's important to note that Ponyboy is initially reluctant to have his hair cut, but he eventually agrees because he trusts Johnny's judgment and understands the need for this drastic measure.