At higher frequencies (such as mmWave), the signal becomes weak due to propagation losses. To compensate for these losses, antennas are made into arrays, which offers high directive gain and beamforming capabilities. Narrow beams decrease interference by directly targeting of users. This way cellular networks have larger capacity and efficiency.
 
However, there are a lot of challenges with beamforming. For example, even if an array behaves well at 0-degree scanning angle, its performance might significantly decrease at 60-degree by increased coupling between elements of an array, high side lobe levels, front-to-back radiation and so on. Designers need to simulate their arrays to minimize these effects.