Some tilings are not seamless and have distinct edges. If their hard edges do not fit your design, this tiling parameter can help soften them.
The name edge blending pretty much describes what this tiling parameter does: it blends the edges so the sharp edge disappears.
It works especially well with detailed drawings and photographs, often less so with clean illustrations.
Currently the following tilings support edge blending:
First, select a tiling that supports edge blending, then move the "Edge Blend" slider to add blending.
Crucially, you can still edit the pattern with edge blending applied! The trick is selecting a sample of the source image where the blending looks natural, and applied just the right amount of edge blending: