Yes it is only for the visual effect in The Cursed.knorke wrote:iirc in Cursed it is just visual, just like in SC:BW (beside being only place for zerg to build)
It should not be very hard to used some existing gadgets to do some stuff that was mentioned above. These should work:
"Ground Decal Fix" by CarRepairer: it would need some modifications, but it will spawn a non blocking, invisible fakeunit that has/creates a ground decal.
"Aura" and "Attributes" by CarRepairer: you can use the fakeunit as aura-caster (healing allied units, making them faster, take less damage, ... whatever you want) with a range that correlates with the visuals.
"Tech Trees (with count, range, sign, ...)" by zwzsg: give all buildings a prerequisite that is only provided within a certain range around the fakeunit. Thus you can only build on the ground decal.
All gadgets can be found in The Cursed btw. The needed modifications should be minimal. A growing creep could be simulated by replacing the fakeunit with another one after some time.