As I've seen around, ever ybody uses borders in their avisynth scripts. Personally, I never add borders, becuase of the excessive "dark matter"

on the left and right sides of the video. I prefer having borders only above and below the video. So basically my questions are:
1. What do borders exactly accomplish?
2. Will it reduce the file size of the encoded product?
3. Is it
absolutely necessary to have them?
Some of these questions may seem "dumb" to some people, but I just wanna know!
Any help is appreciated!