[Kerbal Space Program: Breaking Ground Expansion - Official Gameplay Trailer]

1 hour ago, MechBFP said:

Why is a telescopic piston with a hinge on it not good enough? Does the exact same thing doesn’t it?

It isn't really telescopic, it's quite bulky (including the smaller version), the reason it isn't good enough is due to the wobble of the piston, convenience (easy movement placement, and creation.) and a few more. But yes it does do the same thing.

[Kerbal Space Program: Breaking Ground Expansion - Official Gameplay Trailer]

2 hours ago, StrandedonEarth said:

Have you seen the custom KSP arms built so far? The part count does not go up much, plus you can build it to be able to move exactly how you want it to. Plus, having an arm with multiple joints would be harder for the devs to configure the multiple joints. This way it’s one part, one joint to configure. 

No matter how they designed an arm, someone would be wishing it was different 

We are talking about a sliding part, not a multi-joint arm. 

[Kerbal Space Program: Breaking Ground Expansion - Official Gameplay Trailer]

8 hours ago, _stilgar_ said:

It can be build using a mix of hinges and pistons. 

Benefits of having the part:

1. It would keep part count low.
2. Having a dedicated part will do exactly what you want.
3. Having a single block to do exactly what you need would be simpler with a lot less configuring.
4. It would be convenient to place down one block rather than build a big contraption.

[Kerbal Space Program: Breaking Ground Expansion - Official Gameplay Trailer]

The only thing missing is a sliding rail.