Multiple software engineers working to solve the same problem in isolation will arrive at different solutions. Some may work better than others, but that's irrelevant; what's important is that these different engineers will have all achieved a complete understanding of the problem and its solution, and when they collaborate on a new, more complex problem that builds on what they've already done, they each bring their own unique perspective to the table.

This is why side projects are so absolutely crucial to a software engineer's mastery of his or her craft; by working through a problem alone, the engineer not only gets a better understanding of the problem, but is also able to apply diversity of thought to a team's problem set.