I wouldn't use a whip on a aggressive horse. It could only make them more aggressive and harder to work with
I wouldn't say they are "handy" for aggressive horses. You shouldn't need them on the ground with your horse and under saddle they are only used for an extension of your leg/arm like said above.
I simply use a whip for an extension of my arm. This is because without the presence of the whip my horse won't listen. But I still never hit my horse as punishment
Some horses are scared of them, it's a punishment, but for me my pony thinks it's great so she can go fast!!!
I use crops/whips only as an extension of my arm/leg and i hardly ever use them anyway ;p
Bc when u use a whip u use it to hit a horse to tell it to knock off whatever it is doing and horses tend to get scared of them so they calm down Bc they don't want to get hit