Starting from 2G, cellular operators used polarization diversity to enhance network capacity. The very first idea was to use horizontal and vertical polarizations, but ±45 degree - otherwise known as slant polarization - has proven to be more efficient.

• Horizontal polarization can be easily blocked by buildings and other big structures, so it cannot be used as it is.
• Vertical polarization shows the best results for the network on long distances, but the coverage is very poor on short distances, due to mostly horizontal position of user terminals.

So that's why we have it: slant polarization is simply the best way of achieving good signal levels and dealing with interferences from nearby antennas.