Originally Posted by HookwitdaHookah
I always assumed the speed bag was to train you to keep your hands/arms up so you don't drop them and get KTFO. I mean, you have to hold your hands up high and hit that thing rapidly for 3 minutes straight and for 3-4 rounds. It improves your arm endurance so you don't drop them when you're tired. But since amateur boxers only fight a few rounds, it's probably not as useful for them as oppose to a pro boxer fighting 12 (and used to be 15) rounds.
Heavy bag is for punching. Maybe you can improve your punching endurance (being able to throw more punches) by hitting it repeatedly. But I don't see how that will train you to hold your arms up in front of your face.
How you use a bag is up to you and your discipline. If you use a heavy bag with your hands low? You are doing it wrong. You should be "fighting" a bag exactly like you fight a person unless you are doing a pure punching endurance session.
I do see the endurance benefit that you speak of though.