The supposed purpose is to allow the muscle to be equally worked through all ranges of motion.Each cam for individual machines are designed for the specific exercise for which the machine is meant to be used.
The basis for a lot of common powerlifting practices are questionable though, which includes accommodating resistance.
Training tools such as medicine balls, kettlebells and resistance bands also provide constant resistance.
Medicine balls and kettlebells can often be used in place of free weights, and resistance bands can help to strengthen all of the major muscle groups in the body.
In my opinion only a small handful ofinnovative devices have been introduced, and most of those during thelast two or three years.
Whether they have improved the effectivenessof strength training is often hotly debated.