In SPV5: High Tension Exercises for Muscular Growth I discuss how high muscular tension is the primary factor for muscular hypertrophy. The tension has to be high enough, but also long enough. The optimal time under tension to build muscle is 40-60 seconds. In other words, the set has to last that long in order for you to grow.
The problem is that when people lift, they stick to the traditional range of 6-10 reps. Their sets last no longer than 20-30 seconds.
Timed sets allow you to focus on accomplishing the goal (optimal time under tension) as opposed to the accomplishing the task (hitting a target rep).