Sometimes it is necessary for Traincontroller to force a SIGM20 signal to red. This can be where a train needs to take a route that is blocked by other trains; at crossings for example. The SIGM20 doesn't know that Traincontroller has reserved track, so can't take account of it.
The SIGM20 still sets the signal to red/amber/green based in the track ahead. All this change does is to set the signal red for other conditions that haven't yet affected the track ahead, but may do so soon. I suspect that most signals will need this setting eventually, if it is to become safe for a human train driver to drive round the railway.