Daka

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.

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

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.

The only thing missing is a sliding rail.