I agree that a good purpose for reading will have the Why along with the What (the goal). Putting a little effort in making the What more specific is helpful.
If after a while you notice your goal has change, then change it and PR again. It's fast. It's easy, and it's fun. 
The Why help set a priority for the problem or PRing so that your inner mind will work on it, as apposed to one of the other many problems it has to deal with.
Good luck,
Iam2 
