few things that come to mind quickly (but keep in mind, I've never run anything other than a k-boom, so I don't have full background on owning/running other cranes).
-Lower truck height when packaged up, can get in and under trees, lines obstacles better.
- Can have a dumping bed. Or better yet I hooklift system to change beds and use the truck for much more than just a crane.
-Don't have to keep blocking up on slanted grades, up to 5% off is fine. (can go more if you make smart light picks

).
-hydraulic power at the end of the boom, can run many types of hydro tools.
-I thought mine was way faster moving than stick cranes. The ones I've seen anyway. Fast enough I would never want to rotate full speed with the boom extended very far. Or lift or lower my last knuckle full speed is too fast.
- can set up near lines, reach under lines, and knuckle upward to take a higher pick than the line.
-lot of knuckles to get the right angles to send the boom into say a container or into an open window in a building.
more might come to mind later.
yes, they are expensive.
IF, our truck was just a k-boom, I don't think for the money I would have it. I would have an AT crane instead. But, since it has the k-boom and palift hooklift, our truck does multiple duties all the time.
Also, when using the truck for flatbed, log bed or chip box duties, the k-boom comes in handy when certain things pop up. It's there, tucked away waiting to help.
Like just on this past thursday, I delivered two holly trees to a site to be planted. I thought I would just put the flatbed on the ground, take the handcart and roll them into the yard. when i saw the yard, it had a steep hill in the yard and I could see that the two of us were going to have trouble pulling that cart with 300lbs or so of root ball up that hill. So, the k-boom went into action, set the trees on the top of the hill, easy as can be.
I should update my old k-boom thread with new pictures.