why knuckle boom trucks?

 A knuckleboom crane appears similar to a standard crane, except that the boom articulates at the 'knuckle' near the middle, letting it fold back like a finger. This provides a compact size for storage and manoeuvring. 

A knuckle boom truck ( mobile crane) is also known as a picker crane, loader crane and articulating crane. 

It resembles a traditional straight boom crane but it actually has two booms that articulate at the knuckle in the middle and resembles the action of the human finger.

Knuckle boom cranes are known for their ability to maneuver in tight spaces and their compact storage size. It has the ability to lift heavy objects and place them inside a building through small spaces, like a doorway.
 

 
Another advantage of the KBC is that they are lighter than conventional boom cranes, and can then be mounted behind a truck cab, leaving room for payload as well.

Adding to their maneuverability, the cranes can be controlled in a variety of ways by an operator. They can be radio remote-controlled, controlled from the ground and from a seated or standing position.

 An articulating boom lift, also known as a knuckle boom, has multiple sections, with “joints” in between them. ... Like the telescopic boom lift, thearticulating boom lift can travel over uneven terrain, but it doesn't have the same horizontal reach.