On my twin toroidal power supply, I had some problems with supply to the house dropping below the legal voltage
Once it dropped below 200V, which was unacceptable and caused all sorts of issues.
so I fitted one of these APC Line-R 1200VA Automatic Voltage Regulator - LE1200I
It solved that problem instantly
Since then western power faitted a voltage monitor and within a few weeks of seeing the readings, dug the road up and fed our place from the other end of the line and voltages are all within spec so I guess that bit of kit is redundant.