The heating is intended to kill proteases, nucleases, etc. and to wash away preservatives or whatever is there in the bag that might interact with your protein or ruin things in an unanticipated manner. The buffer -I am guessing now- is just to maintain a friendly pH.
By the way, the bag is not 'activated' by the procedure. I have used dialysis membranes directly off the roll after just a brief rinse/dip in water and they work just the same (meaning the protein did not degrade, and the buffer was changed).