Some do that because they think it looks cool...Called "Bull Horn" Mounting...
Originally, guys did that because of clearance issues so the whip wasn't hitting things where they were delivering or like on Rock haulers the antenna wouldn't hit the bucket or loaders.
If anything it would hinder the antenna's performance...If you don't have to do it, I wouldn't recommend it.
Actually, I think it looks lame and I personally wouldnt do it. If a loader hits my antennas he should look for another job!!