It's good to incorporate background legends and mythology in your campaign world. Stories that aren't necessarily true, but that characters within the world know as part of their cultural background. Most of the time this is just detailed world-building, but sometimes it can provide a hook for the players and the NPCs to hang important plot stuff on.

This is the basic goal of campaign world building: providing hooks. You need to provide plenty of hooks, because your players will pick and choose only certain ones to hang things on. The thing is, they'll have plot elements in their heads, just looking for these hooks. And if the hooks aren't there, they won't get things attached to them and any potential story can unravel and end up loose on the floor. You can end up with a world in which the PCs wander aimlessly from one encounter to the next, fighting monsters or villains, and then going in search of the next challenge, but without any over-arching theme or story arc.

(This is a risk - we're not saying it will happen. Diligent GMing is a multi-faceted discipline, and you can weave a story out of all sorts of threads if you spend the time and skill doing so. We're just suggesting one possible tool to make it easier.)

But with hooks in place, the players can pick them up and use them to weave their own story fragments. Build their own legends. Become a part of the world-building. This achieves two things: lightens your load as the GM, and gets them intimately involved in the world.

Which of these two ends is more important may depend on your GMing style, but most GMs will agree that at least one of them is worthwhile.

Transcript

Anakin: Obi-Wan, I want to thank you.
Obi-Wan: That's... unexpected. What do you mean?
Anakin: I've worked so hard to hide my true nature that I hid it from myself. You've helped me to see what I really am.
Anakin: Like this lava.
Anakin: It was here all along, but the shields had held it at bay. Protected the things we'd constructed on this world from being consumed.
Anakin: Now you've broken down the barriers, and the artifice was found wanting. There's no need for a facade any more.
Anakin: I am the Sith Lord.
Obi-Wan: But the Sith are a legend!
Anakin: I am the legend.