I am no expert on 5/3/1, but I believe it's all to prepare for that final set that is a near max effort. I know one thing even now hours after the workout my chest is pumped and throbbing, I can feel the blood going through it as I am typing this.
If you go back to my first week you can see my comments where I think it's the easiest workout I have ever done, and it is reasonably easy, but it seems to work so far.
Ask me again in a few months to see how it's going, week three of month three I will be testing my one rep maxes for each lift again, and I am hoping they will be better than my signature, feeling pretty confident at this stage to be honest that I will smash them.
This is only a three month cycle, at this stage I am thinking that after my new 1 rep maxes are set I will have a week off training and then start another three months challenge using my new 1 rep max and different assistance exercises.