It's about being a dreamer. It's about being a risk taker. It's about being overconfident in realms that may be familiar. It's about being so passionate about what your doing, it becomes infectious.
Realists are better fit for project managers at large corporations...
No, that's being an idiot.
A successful entrepreneur - as opposed to someone who loses their shirt - may be a visionary and a dreamer. But they take calculated risks. You have to know when to persevere, when to pivot, and when to give up - especially if you're playing with other people's money or your life savings.
Yes, vision and confidence are important, in order to bring employees, investors and customers on board. But unless they are grounded in reality, you will fail horribly, unless you are very, very lucky.