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.
Originally Posted by bald_head_slick
You "could" improve arm endurance using it, but why not a heavy bag?
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.