Can some of you more knowledgeable people please explain in what
circumstance(s) would it be of benefit to add a travelcard to an Oyster
PAYG?

I always though that Oyster was valid throughout the London Underground
network and in all zones and that you are charged the minimum rate for
whichever journey you made. Is this not the case or am I missing something
somewhere?

Regards,

Clint