Readers are not given a declarative statement that expressly tells us why rehearsals of the executions are done, but it is clear that the rehearsals are there for a variety of reasons. The rehearsals function as a way to train new guards to what will happen. The rehearsals also function as a reminder to the seasoned guards of exactly the procedure that they are going to have to do. The executions are seen by people, and the guards want things to look smooth and planned out for those people. The rehearsals also allow the guards to practice moving through their steps so that everything at the actual execution is step-by-step procedural and muscle memory. It isn't only a single guard walking the prisoner down. It's a team of men, and they need to know how and when to move and where and when to do it. Finally, the rehearsal will point out to the guards potential problem spots or faulty equipment.
The first rehearsal went well, and so did the second. Percy performed better than I could have hoped for in my wildest dreams. That didn't mean things would go right when the time really came for the Cajun to walk the Mile, but it was a big step in the right direction.