I personally haven't used the tools, but well known power lifter Mike Tuscherer does. They help analyze the power you apply to the bar. When you combine a tool like that with Compensatory Acceleration Training, it can tell you how well you are accelerating through the lift as it becomes more mechanically advantageous.
Tools like that are most useful when comparing a string of training sessions. Let's say you train like a Sam Byrd where you stay at the same weight and try to minimize the time it takes to lift it for 5 sets of 5. Sam first improves the rep speed using CAT until there is too little change in that before he minimizes rest times. The tool will tell you whether your bar speed and power production are increasing or staying fairly constant.
The more dialed in your technique, the more useful the gadget becomes. If you don't train like Sam Byrd, and you really don't care about CAT or bar speed, then these units aren't going to be of much use to you. In short, you may have to change the way you train to make use of it.