I think it makes visually more obvious the separation between the placed pieces and the empty spaces (I guess that's just the definition of a border haha)
Possibly non-trivial to give borders to the outer borders of the piece squares without giving borders to the inner borders of the piece squares, but maybe it's not bad to do both outer and inner.
Also note, blokee.mattle.online does not have borders, but distinguishes between taken squares and empty squares with a "shininess" effect to give the taken squares depth while the empty squares look flat cuz they don't have any effect.
I think it makes visually more obvious the separation between the placed pieces and the empty spaces (I guess that's just the definition of a border haha)
Possibly non-trivial to give borders to the outer borders of the piece squares without giving borders to the inner borders of the piece squares, but maybe it's not bad to do both outer and inner.
Also note, blokee.mattle.online does not have borders, but distinguishes between taken squares and empty squares with a "shininess" effect to give the taken squares depth while the empty squares look flat cuz they don't have any effect.