If you are a solo entrepreneur or a founder at some point you have to explain why you are best person to deliver the service. Knowing who you are helps with that.
Knowing who you are (and who your aren't) speeds up decisions and reduces time between having an idea and taking action.
Clarity on who you are and what you are about makes finding the right partners easier and gives them compelling reasons to say yes to working with you.
Wondering about who you are is an energy drain. Being confident about that frees you from all that worry and distraction and allows you to move forward faster.