I don't know about you, but I can't remember the last time I succeeded in doing something entirely on my own. Although my recent forays into wallpapering haven't turned out too badly.
In my career, the best outcomes have been achieved when I've had a team around me - either virtually or physically - to bounce ideas to, or to carve up tasks so that they're done quickly and efficiently.
You have more fun working with others. Often you end up doing things you hadn't even considered from the outset.
Henry Ford: "Coming together is a beginning; keeping together is progress; working together is success."
And then there's diversity. When you involve people from other backgrounds, you're able to see in a new way. There are different filters, different facets and different translations of your key messages that suddenly burst forth. They were never visible before! Now you're appealing to a whole range of audiences.
Here's an obvious one: personal strengths. Everyone is really good at something, but maybe not so good at other things. I know someone who is a fantastic creative designer, but ask her to write copy, and she loses her confidence immediately. That's why a strong team can field a wide range of talents, so you get the best from everyone.