I will spare you for asking such a silly question. Everyone was a noob at one point :nana:
The lens is used to magnify the light from the LED and focus it into a tight beam, and small hotspot with little to no flood light. This is great for spotlights, and long range flashlights. It is not recommended to be used with a reflector, as that would create flood light, witch for most applications would be considered to be just a distraction.
Here is my favorite aspheric mod on a flashlight. Made by Pepko.
Here is a great link, it demonstrates how exacty curved glass can change the direction on light. A member here on CPF originally posted this, but I cannot find the post.
Like SmurfTac already said the aspherical head really tightens the beam up and maximizes it for nothing but throw, with little to absolutely no spill. Use the CPF search box and type in "best aspherical lights" to see examples.