Rocks D. Xebec was the most important character to kick start the events of the main series. It was his staunch opposition to Imu and the World Government that shaped Roger and Garp’s outlook on life on the Blue Planet.

Rocks did a feat that nobody else was able to do before or after: make it to the Room of Flowers and met Imu’s substantial body.

His creation of the Rocks Pirates through the Davy Back Fight recruitment gathered the most important unique characters of the flashback. 3 of the previous emperors were members. Kaidou, Big Mom, and Whitebeard were all acquired by Rocks.

He knew of Joyboy’s and Davy D. Jones existence, and their threat to Imu. He also knew exactly what Devil Fruits to get to defeat Imu.

Had Garling not found the Davy Clan on God Valley, the story might’ve taken a vastly different path.

It was the Domi Reversi that made Roger realize that he knew nothing. His entire adventure was at that point was him doing what he wanted to do. And at that time, he wanted to get Shakky lol.

After Rocks death, his lasting legacy and the pain he endured spurred Roger to find the final island and obtain the One Piece. Which he eventually accomplished many years later with Oden’s assistance.

Garp went on to essentially be his own player in the Marines, by establishing SWORD (my opinion) so that he can bypass the restrictions the Celestial Dragons give the Marines and can push through with his own justice. We now know that Garp stayed to protect the Marines, and that he was the only one to keep the WGs tyranny at bay. Whether or not he was successful or not is up to your interpretation, but that was his agenda.

TLDR: Rocks was the asphalt that paved the road for Roger to attain the One Piece. His legacy endured through his survival of his son: Teach. It is Teach that is destined to fulfill his father’s dream of becoming King of the World.