The Mechano-Tank Attack Target is just a target for you to aim and fire at when you are in the Shooting Mechano-Tank.
Note that you do have to actually hit the target. While the pellets spread a good bit, you CAN miss the target entirely if you are too close.
The Mechano-Tank Attack Target is involved in the following quest of the Operation: Gnomeregan quest chain:
- Patch 3.3.5 (2010-06-22): Added.
Event activated September 7/8, 2010