I always thought Dio's influence was about more than just being a strong villain. He's like a corrosive element poured into the lives of every Joestar. His very existence warps the narrative gravity, pulling people into orbits of obsession or revenge. Look at Polnareff—his entire arc is a direct response to Dio's cruelty, even before he meets Jotaro. The conflict often feels less like 'good vs. evil' and more like a world dealing with the lingering poison Dio left behind, years or even generations later. That's why Stands manifest; they're almost a metaphysical immune response to his presence.
He also sets the tone for the rivalries. They're deeply personal, not just power clashes. Jotaro's cold fury, Joseph's strategic desperation, even Johnathan's tragic friendship-turned-hatred—all are colored by Dio's particular brand of theatrical malice. He creates conflicts that are emotionally messy, which makes the victories so much sweeter and the defeats absolutely brutal. The story would just be a series of monster-of-the-week fights without that personal, hateful core he provides.