Well if Dio never put on the Stone Mask, Jonathan and Speedwagon would have never been aware of its true power, thereby never knowing about the vampires. Will Zeppeli would still have been hunting it down, but I doubt he would have made as far as he did without training Jonathan. And since Dio never attacks Jonathan on his honeymoon, Lisa Lisa would never learn Hamon and live a normal life, meaning Straizo never gives her the Red Stone. Fast forward to when the Nazis find the Pillar Men, they won't be able to defeat them, causing Kars to find the Red Stone, become the ultimate life form, and destroy humanity!

tl;dr: DIO INDIRECTLY SAVED THE FUCKING WORLD

he only needed a Spiral staircase, Rhinoceros beetle, Ruins street, Fig tart, Rhinoceros beetle, Via dolorosa, Rhinoceros beetle, Singularity point, Giotto, Angel, Hydrangea, Rhinoceros beetle, Singularity point, and a Secret emperor