TB,
there are a couple of reasons to monitor Guard, one is if you are for some reason not listening out on the correct frequency for any reason, this might be because you have turned the volume down whilst doing a PA, selected the incorrect frequency and gone out of range of the old one or you could be a VFR aircraft about to stray into CAS in these cases ATC or a intercepting aircraft will try and contact you on Guard.
Another reason to monitor is to listen out for any other aircraft in trouble, it may be they are on the ground or in the sea and unable to contact ground services direct, an aircraft at height will be able to pick up their tx and relay it on to the emergency services.
