Thanks for the photo Raichase.
As for why any train needs a target plate, if anything it makes more sense on a semi-permanently coupled sets like the Waratahs, Millenniums and Tangaras, where the set number is essentially always going to refer to the same set of 4 or 8 cars.
Actually, I beg to differ - it makes less sense on anything that's permanently coupled into sets. May as well permanently put the set number on the train in some form or another, because it's not going to change. In the case of the Tangaras, cars can be removed and added, there are quite a few mismatched sets about. In the case of the OSCars and the Waratahs, set numbers could (in theory) be permanently applied. They all come from the same depot, and are marshalled in fixed sets, why bother with a removable target plate?
Target plates are more important on sets that can easily be remarshalled, with interchangeable cars, like the S/K/C sets. Obviously K/C sets work in pairs, but they are not fixed 4-car sets. As they can be at different depots, they need removeable target plates. Just because the K's have settled at HMC and the C's at FMC, doesn't mean they were always that way.
I would like to know why it is necessary to have a set number in addition to car numbers though, AIUI the set number is not what shows up on a signallers computer screen, and I hope to hell that maintenance is recorded and undertaken by car number and not set number.
Identification purposes. If a driver identifies their train as being in 2 road, Set S1/S8, they can easily check the train is the correct one. Ditto that a signaller would prefer to hear that the defective train is T28, and not a bunch of car numbers. It's also easier for train technicians, transit officers, police, etc to identify a train coming into a platform by set number than by car number. Troublemaker on M4, 3rd car? No worries.