I used to hit them back when I trained , there a good tool but the main reason I feel speedbags are for , is to teach people to constantly change angles , and not plant their feet which teaches beginners that there is more to fighting than power and speed. But technique , movement , and tactics.
Speed bags , will give you a good workout regardless of whether you start off correctly or you hit them just how you've seen they should be hit , but will give a greater workout if you slowly progress focusing on constantly moving and constantly doing 0-90-180 degree sweeps to keep your opponent off balance and train you to remember not to stay in one spot and be able to counter you opponents planting his feet , and or throwing power shots.
If you do not have a speed bag my suggestion is to get a double end bag , their easier on a beginner and give a better muscular , and cardiovascular endurance workout... Along with being less boring overtime , and carries over more to real life situations in real life fighting. Its a more functional training tool , and keeps rust away since its correct punches which a speed bag is not.....
If you already have a speed bag , and do not want to add a double end bag to the mix. Then my suggestion is just start slow. and focus on movement and footwork the entire time you are hitting the bag maybe 2-3 second stand stills at the most when hitting the bag , before you move again. Start with the basic 3 left , 3 right , 3 left , 3 right combinations while moving around at slow speed... Once you feel you have no messed up with your balance , missed the bag , not hit the bag off in a bad direction , your footwork did not cross you up , or throw off your focus , and you did not have to think about your movement , everything was perfect for arond 30 minutes of speedbag work.
THEN , speed up a little bit , and repeat the process before speeding up again.