It is a technique to improve the horizontal radiation pattern of a high power transmitter station.
In a high power VHF or UHF station, usually the antenna system is constructed to broadcast to four directions each separated 90° from each other.
So the directivity of the antenna system resembles a four leaf clover.
One popular method to solve the problem is to skew the antenna panels symmetrically around the central axis of the mast.
[1] Usually a skew of λ/4 gives the desired almost-uniform horizontal radiation pattern.