It depends. If you have vision issues, of course a prescription will be necessary. But I think there is no need for it if you have a perfect vision. You’d better confirm it with a profession optician though.
I don’t think it is quite necessary since the point of wearing them is to change the eye color. I know many girls wear this type of lens just to make add more charm.
It is absolutely necessary cause you still need to be fitted for the lenses even if you don't need vision correction. This is because contact lenses are medical devices and wearing them can affect the health of your eyes.