Some people need them, but some just like to be seen using one. If you have street cred, it means that people of your age, usually young people, approve of you and accept you. It's about gaining acceptability - you share their views, their sense of fashion. It's peer pressure, too. You've got to be careful what you wear.
Trainers are a good idea.
But make sure your trainers are a well-known and fashionable make. Make sure you wear a designer label. We've been talking about street cred. To have street credibility, first buy a mobile phone. Then don't forget to wear trainers, but make sure they have a designer label.