As a addon developer it would be very useful if you always had the ptr around to test various constellations in the game, or just to verify bugs.
Therefore it would be useful if you had
a) templates for various stages ,levels so to say
b) the option to define the state of questing for that character. Change the completed quests so to say, to make sure that all branches and possibilities are tested to avoid people getting stuck and help fix exotic edge cases.
Example the war campaign , some chars come later and need the fast track, some came earlier, some voted for Sylvanas , some against.
This can break stuff that relies on figuring out the influence of certain decisions.
What you get is better bug reports and a vibrant addon community