For a fully runtime procedurally generated world, not that much.
If what you have is a collection of blocks that can attach together randomly (like rooms with doors as connection points), you could implement a runtime hash that handles the selection of these rooms based on a seed, and these blocks could have data layers that can be activated (for missions, specific events, etc).
Another way data layers could be usefull is if you have hand-crafted areas in your procedurally generated world, such as cities or anything designed by hand. Then, you could have data layers to enable/disable things, etc.
Hope that helps!