Because barbarian cities grow just like everyone else, their warehouse gets larger = more resources for you to pillage. To prevent an enemy, (or friend) to conquer your fave nearby barbarian cities I use the Ballitas to periodically take down the city hall, barracks, forge. This makes them less attractive and keeps their points small so no one wants to conquer it. I start to do this while I farm after they reach about 3000 points.
I've been in a situation where I've needed resources and haven't been able to farm because all my barbs were taken over by everyone. I crop them instead of conquering them bcs they are good to have in an emergency when all your other cities are out of resources. Its also a strategic advantage to having readily available resources 15 min away when your enemy may not have any. I also sometimes ballista the barbarian cities around any potential enemy I'm planning on taking over. I hit their warehouse so there will be no resources for them to pillage while I'm taking them over. Cruel but effective. The barbarian will re grow the warehouse, your new city will be added to your trophy collection. Its win-win.
** Addition **
Barbs only grow to a certain point size naturally. They get no larger then 11k from what I can tell. So this is why Barbie Cropping can come in handy, BECAUSE they only grow to a certain size, and because the growth is completely random, if you find that your nearby barbarian is being a jerk and focusing on lets say Market or Forge instead of more practical farming buildings like Warehouse or the resource buildings... Use your ballistas to knock down the unessential buildings, this will stimulate growth because a barb will continue to grow until it reaches that magical 11k point level... Granted it is random and he may just rebuild his forge, but there's a good chance he will build the buildings you want him to grow as well.
Another good use for balllistas when it comes to barbie cropping is to specialize your farm - have you ever wished while farming you can JUST haul wood?? Well if you take the time to ballista iron and clay buildings in a nearby barb, the wood production will max out much faster so when you farm that barb you will haul significantly more wood then the clay and iron. I sometimes do this with three barbs, keeping specific resource in each. It is indeed a bit of work, but well worth the effort. The trick is remembering which barb has which resource!!!
