“Players don't win you trophies, teams win trophies”. A team works better when they work together, no matter whether the goal is winning the World Cup or creating amazing websites.
Any development team, whether large or small needs to ensure that it works together as a cohesive unit in order to produce the best possible output. This can mean ensuring there's consistency in the code being written. Ensuring everyone stays up-to-date with the fast-moving pace of the web industry. As well as the ability for any project to be worked on by one or more team members at a time and have the ability to be handed over if a developer takes time off.