My company mandates monitoring guard in case of loss of comms. As the previous posts added this will be the frequency ATC will try to call you on when you go lost comms because they forgot to hand you over. On many frequencies around Europe, you wouldn't believe how quiet the frequencies can get, so you wouldn't even necessarily realise you have lost comms until you're out of range.
ATC may well have forgotten to hand you over, but isn't there a point at which you should be thinking to yourself, "I need to be changing frequency around here?"