I’ve searched but nowhere does it say what they’re for, only that they exist…
I’ve changed border size on some sprite in the sprite editor but it does nothing visually to the sprite, and it’s not a collider of any sort even though its colored green like one.
Sprite borders are used for ‘9-slice’ scaling. Anything in the border area won’t be rescaled. It’s most relevant for UI and other resolution dependent elements. If you don’t scale your sprites you won’t notice a difference.