Christ on a crutch, I was only asking if they were neccessary and what purpose they served.
Necessary ?
Well as they provide the two threaded holes for the fixing screws then they serve at least one purpose. A couple of wood screws into Rawlplugs will work if you take the time and effort to get the holes in the right places. If not you screw the plate up tight and it cracks.
Safety
If the hole in the wall is a bit crumbly then the wood screws might pull out when you are pulling a stubborn plug out of the socket. Not safe to have a socket hanging on its cables.
Without a back box bits of wall might over time come loose and make contact with live parts in the back of the socket. Not a problem if your walls are bone dry. The back box takes care of that problem.
And without a back box the top of the hole might fall into the space behind the socket leaving a hole above into which little fingers might probe and find a the tops of a live terminal screw.