Butt sections are stiffer. There's a rule in fly fishing, heavier turns over lighter. In order to turn the leader over you need a taper, just like flyline tapers.
I know someone will say they use a straight piece of mono with no problems, but they're throwing a fly heavy enough that its momentum straightens the leader and not the leader carrying the fly. A good rule of thumbs is if you can throw a fly a little distance with your hand, you probably don't need a tapered leader, if not then you need the leader to carry the fly.