As a general guideline, you want to use chains that are roughly 10% of the barbell weight (bar + plates + collars). So if the loaded barbell weight is 100 kilos (220 pounds), each chain should weigh 10 kilos (22 pounds). But don't get too detail-oriented—even a 15% chain will do.
source: Bodybuilding.com, How and Why to Train With Chains
Weight lifting chains - also called powerlifting chains, barbell chains or bench press chains - are the best way to add a "resistance curve" to your lifts. As the length of chain lifts off the floor, it adds more weight to the bar, so you're lifting more at the top of the lift. You have more leverage at the top of a lift, so chains help even out the amount of effort required through the entire range of motion.
Chains can also be great for speed training. During high speed reps, the momentum can cause you to lose control of the bar at the top of the lift, or at least force you to stop the bar's momentum. Chains add resistance at the top, slowing down the momentum, so you can get really explosive without worrying about throwing the bar up out of your hands when bench pressing.They can also be used to add to the bar for landmine training, as pictured.