Never been a fan of the DBL end bag, nor the speed bag for that matter, but had to do it when I was boxing.
In one word: MOVE
The DBL end bag teaches you hand/eye coordination BUT you have to move around it and evade and such.
You are not fighting it nor are you actually moving and hitting the same way as you would in a fight BUt you are developing some good coordination and timing and footwork.
It also breaks the monotony of heavy bag work.