You don’t NEED it but it really depends on what you’re making. For my prinzregententorte, I did not use a simple syrup because the cake layers are so thin that the buttercream between all those layers keeps it moist for longer. For my fraisier cake, I did use a simple syrup because the two layers were slightly thicker. If you want to add extra flavor to the cake, a simple syrup is definitely helpful. Feel free to add vanilla or any extract or to the simple syrup (after it's removed from the heat). For example, the coffee soak in my Tiramisu Cake and Opera Cake, the simple syrup in my Cranberry Orange Cake, a lemon simple syrup from my Lemon Poppy Seed Cake, or just a stronger vanilla flavor.  I haven’t tried this yet but I just thought about soaking a genoise with my brown sugar cinnamon syrup and that sounds heavenly.