You ask why they use them, I ask why not?.
First of all the passenger and cargo increase on these busy routes. And also because it is the new flagship of domestic. It is very comfortable and offers high standard, not to mention faster loading/unloading times with dual airbridges.
And yes QF
will use the 201's on domestic only.
The new 300's will come with Total Entertainment, and (hopefully) new business class seats. I would think that the new 300's would replace 767's on international routes. and the 767's on international duties would be transferred to domestic, and therfore the retirement of the 762's will happen.
So, in the future I would expect to see the 333's on Japan, HK
and some Trans-tasman flights.
It is all part of this fleet restructuring thing and associated works going on, wherby all 733's will be All economy (slimline seats) and 734's with 20 permanent J class seats. 73H's in a 12/156 config and millenium and slimline seating.
Anyway, back to the topic, overall the 200's are used purely domestic because of their ability to carry more Pax in peaks and also because they are more economical therefore QF
can offer more of the cheap seats to compete better with Virgin.