What is the full name of the boiler. Baxi (what model). The pressure switch is set at 3 bar so if the boiler over pressurises this valve cuts in at 3 bar & you will see water drippinh out of the pipe outsie the boiler. The overpressuring can be caused by a number of reasons - the filling loop faulty & or left turned on. Blockages in the system. if the switch was not fitted the pressure if it built up would have to release from somewhere. For example at a pipe os a raidiator connection.