Hi John:
Well, there are different reasons a teacher might use a pretest. One way (I believe the way you describe) is to take a pretest before the real test to see what areas still need work. That works, and does help students study weaker areas. (caution that it souldn't be the same test).
Another planning use is at the very beginning of a course to see what areas may be needed that require more emphasis than previsously planned, so adjustments can be made in the lesson planner to accomodate the tweaking.
Another use is to compare learning progress early… >>>