So the principle is: If the design of your antenna is such that when the signal emitted by one disc reaches the next disc some nanoseconds later, the phases line up (the next disc is adding power because it's exactly in sync with the signal coming off the first disc), each disc will progressively reinforce the signal so the emitted signal adds up to be way more powerful in the exact direction that the antenna is pointed. And likewise for receiving from that specific direction.

Yagi antennas use the same principle, and another poster gave a much more technical explanation of why discs instead of a Yagi or helix, but in case you were asking the more basic question, that's why.