What is it?
Optimism is a way of seeing the world that tells you good will win out in the end.
Why is it important?
When we take the viewpoint that good will win out in the end it causes us to no longer seek win/lose outcomes or see others as obstacles to overcome. We lose significant fear - fear of loss, fear of failure and other fears that cause us to not go first, avoid risk and other limiting behaviors.
Optimists have been shown to live longer, healthier, happier lives than pessimists in research studies. When we are healthier and happier - and when we do not fear failure, loss of sale etc, we tend to perform at a higher level than our counterparts.