At full adulthood - no purpose. This would fit in well with the "age-related niche partitioning" theory on dinosaur growth stages.
Wow... I was thinking of centipedes. Their venomous claws are used like mandibles... But are in fact legs... err... modified claws.
Never knew, there are even tetrapods with this behaviour. Faszinating!
I do like the massive dewlap though.