If you shot every shot perfectly, and the weather conditions were perfect every time you shot, you would not need fletchings!
(I shot a six gold end at 70m to prove this without vanes on my nav's).
However, I could only do it once lol.
The vanes are there to point in the arrow in the right direction (well, steer it actually), when forces other than the "perfect ones" act on it.
I can't actually describe it now I try to, hopefully someone a little more awake than I will be along shortly.... lol.