Nope, not necessary. In a cycled tank it is because it gives bacteria a place to grow. And it is needed to anchor live plants. Other than that, it's just a matter of aesthetics and ease of cleaning. Bare-bottomed tanks are easier to clean, but to me, don't look as nice.
Just make sure if you have decorative stone that they are not large enough to form big enough holes that your betta could get his head stuck in. They love trying to get through openings they can't get through. I lost a betta to that once :(