Why did medieval polyphonic music need rhythmic meter?

In medieval music, the adoption of rhythmic meter was an essential development for the evolution of polyphony. Before this shift, church music was primarily limited to monophonic plainchant, which did not require a synchronized beat between multiple voices.12

As composers began to combine separate, simultaneously sounded melodic lines, they faced the challenge of keeping these independent parts synchronous. Rhythmic meter provided the necessary organizational structure to achieve this alignment. The precise measurement of musical time became an indispensable prerequisite for compositions that followed the rules of consonance, ensuring that voices remained coordinated at points of musical repose.321

To facilitate this, musicians developed rhythmic modes—a system of short, repeated rhythmic patterns—which allowed composers to integrate multiple voice parts effectively. This transition from the fluid, unmeasured flow of plainchant to a structured metrical system laid the foundation for the development of Western polyphonic composition.421