Apart from previously mentioned cases there should be one more reason.
Thales radar system that we use has some filters. One of these filters is level set up. Since unit is app we have to set up levels in such manner to cover our need. Top level is not problem .You may put whatever you want,usually FL300, but lower level is point. If you put FL 0 then reasonable assurance exists that you wouldn't be able to see all a/c especially immediately after departure/missed app point. So it is automatically set up at FL -30.
Also correction for QNH is mandatory to be done. The only problem atco is supposed to do it manually. Each time when QNH changes atco put new numbers into machine. Then favorite French machine calculate itself. No need for further atco intervention
