- The author might have checked that the code works fine (A) or haven't done it (B).
- The reviewer might decide to run the code locally (1) or not (2).
Combination A1 results in duplicate effort. Combinations A2 and B1 are perfect. Combination B2 results in potential bugs.
The MR standard merely eliminating the combinatorial possibilities by sharing information between the author and the reviewer automatically. The end result is that both parties know A2 is the process that the other person follows.