Same reason Hunt has been starting in Origin, with Grant coming on later. Hunt/Walters are the better defenders and do the ground work. Paix/Grant are more creative and elusive, and are able to better take advantage of the fatigue and shape that has been created.
One doesn't work as well without the other, it's like saying the backs and halves create and score most of the tries, so get rid of the forwards in the game because they don't score points.
The frustrating thing is that this is not being picked up by the 'expert pundits'.
They keep talking about bench hookers needing to aspire to become the starting 9... except in the current era there are very few 9's that can churn out 80mins week in week out without needing a break.
If you've got a hooker that can pump out a quality 55-65mins then you need to find them a break somewhere... so do you want that break to cost you 1 or 2 interchanges.
The more important stat is which 9 plays the most minutes out of a pair... because that is effectively the team's number 1 hooker.
In the current era the bench is vitally important due to how fast the game is and the reduction on interchanges.
Interchanges are like gold for modern rugby league and Todd Payten has been one of the big innovators in this respect. When he started Granville on the weekend at prop I straight away noticed that Cotter was on the bench... Cotter usually churns out 55-60mins a game, so to me I could see how he intended to start Granville for 15-20mins, sub on Cotter and then he stays on for the game (1 interchange saved).
Granville is then there as backup if Robson needs a break and can also slot in if a back goes down.. so Granville is now able to fulfil 3 roles in the 17 and also gives him flexibility on the bench in regards to carrying middle forwards. But the commentators are just like "Bwaaah what is Granville doing at prop... and for Cotter!!!"... they've also not picked up that Lolo is effectively playing prop he just doesnt want to wear the 10 jersey for some reason... so Granville was basically starting lock and fulfilling that role of mobile small forward with some ball playing.... so realistically the move wasn't some crazy scheme given Granville already does this when he wants him and Robson out there at the same time.
He also does it with his 2nd rowers. Last year he was starting Gilbert at 11 before subbing Luciano on to play out 60mins... Gilbert then moves to the middle rotation (1 interchange saved, whilst also getting some use out of his 17 jersey).
This year he's doing similar with Neame/Hess, before bringing on Luki/Dunn, etc. Panthers do similar except they rotate 3/4 second rowers across the two positions over the course of a game.
Payten is constantly looking for ways to save his interchanges up and fully utilise his 17 each week.
I dislike Payten, but his use of the bench is definitely something that should be admired... his use of Granville this weekend didnt workout entirely, but I think he left him on for a little too long to save Cotter's energy... he was still out there at the 25 minute mark.
The pundits seem to completely miss what he's trying to do though and in general put far too much importance on who is in the starting 13 each week instead of minute distribution across the 17 as a whole.