What is the purpose of using multiple driven elements connected through phasing lines?

The primary purpose of using multiple driven elements connected through phasing lines is to control the antenna's radiation pattern. By carefully tailoring the phase (timing) and amplitude of the RF energy reaching each element, the resulting electromagnetic waves will reinforce each other in desired directions and cancel out in others. This technique is the foundation of phased array antennas. It allows an operator to mathematically steer the main lobe of the signal or create deep nulls to reject unwanted interference from specific directions. Phasing lines do not prevent harmonic radiation from the transmitter, nor are they used primarily to allow single-band antennas to operate on multiple bands. While they can be designed to lower the angle of radiation, their overarching purpose is complete control of the pattern's shape.