Makes biters less prone to attack:
-Power poles (small, medium & big)
-Rails
-Rail signals (normal and chained)

The above mentioned structures will not appear on the map anymore, this can't be changed.

If they can't get past something they will attack it, which does not apply to rails as they are non-blocking.
If they fire spit at you and you're standing on the rail, it will damage the rail.