Think of your workouts like you're turning on a light with a dimmer switch. I've got these throughout my house.
As you do your warm ups, you turn on the light. But the dimmer switch is down low because it's only a warm up. At this point, you're not using all of your muscles. Here's one of the warm up exercises I did this morning.
Now, as you get into your working sets and those really challenging reps especially toward the end of a set, you're turning that dimmer switch up. This is when you're getting the greatest stimulus. Your body has now activated all the high threshold motor units to lift the weight. Once you've done this, any further work isn't going to turn the light on anymore because you've already maxed it out.
Hope this analogy helps! You probably knew it already intuitively, but I just thought I'd mention it!