Picking a genre first helps establishes the central question and helps in the selection of conflicts that drive your story. If you have mixed genres it helps with precedence. Selecting a genre also helps with your pitch selection as well as your audience and marketing strategies.
Imagine thinking you were the only one. To find out there are more of you in hiding. Still you are the exception since you are the last descendant in your special bloodline. That possesses great power and authority.