One extremely important factor is that the physical length affects the radiation pattern. Roughly speaking: as the EM waves go back and forth through the antenn

One extremely important factor is that the physical length affects the radiation pattern. Roughly speaking: as the EM waves go back and forth through the antenna wire, the electrical current will reach different peak values in different parts of it. The parts where the current is higher are the ones that irradiate most. Sometimes you want to adjust and interfere in this process to create certain patterns of interest. E.g.: an antenna to talk to other ground-based stations should irradiate more to the sides, while one for satellites should irradiate more to the sky. There are many ways of building patterns of interest, and tweaking the antenna length is one of them. On RX/TX antennas, those patterns apply both to TX and RX, i.e., they will pick more signals coming from the same directions they irradiate more strongly. It's a fascinating field. There are many things I don't quite understand yet, and I see some controversy even among professionals, it seems that sometimes they can't settle on a single explanation for certain phenomena.