I was young too and had other things to occupy my time than hanging around street corners.

But I knew others who were less fortunate. Their homes weren't happy places to be or they weren't allowed to bring friends around. In those cases, there aren't many places to go. Cafes (or cheap restaurants like McDs/KFC) don't want groups sitting around unless they are spending money. Leisure facilities often shut around 8. Cinema is expensive. Sitting on a wall chatting with friends is free... and the price of a joint to pass around (the smell can often be stronger than the amount there warrants) is cheap and probably does less harm than if they were buying cheap booze.

I walk past large groups as a single woman and it's easy to feel intimidated as we generally only hear the bad stories and assume they are gangs out to rob us. Often if you actually look them in the eye and smile or say hello, you'll get an embarrassed smile back.