Because this way I get side shift. Whole boom assembly and kingpost moves left-right so you can dig next to a wall, fence, any small space. Other advantage is that you can fold the boom as close as possible to the tractor so you get more manuverability, stability, tractor can travel faster and up a steeper slope. Here, a backhoe with swing down stabilizers is a rare thing...
Here is a video of CAT backhoe showing advantagaes of this system
Here is a video of CAT backhoe showing advantagaes of this system