As Substance Designer documentation shows, the FXAA node is useful to remove jagged, pixelated edges on shapes. Sometimes the use of this node isn't easy to see so I added here some images of a real project were I used it and you can see the before/after.
In this scenario I was getting a very subtle but deformed edge on the marked area, so I decided to use the FXAA node to see if it could improve the results.
After the FXAA node.
Before the FXAA node
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