I think the reason is pretty simple. If Wells had never taken Tempus back in time, Tempus would never have tried to kill baby Clark. And if he hadn't tried that, Clark wouldn't have gone back in time to save himself. And JOnathan and Martha might not have found Clark's spaceship in time - remember, baby Clark landed in the afternoon, but was found in the evening after adult Clark caused the streak of fire leading the Kents to investigate what landed in Shuster's field.
Am I making sense here?
