A bar takes the ab rollout to the next level. Try a wide grip to hit the chest and shoulders as well as the abs. Have a super strong core? Narrow the grip.
Grip width can alter the emphasis of the working muscles in the ab rollout. Using a wider grip will work the chest and shoulders to a higher degree, as the arms will end up in a less stable position at end range.
The abs must work hardest with a narrower grip; it keeps the arms longer, which allows the bar to travel further away from the knees or feet at end range (more distance to bridge and maintain proper position.)
With mechanical drop sets, instead of reducing the load, you switch to a new but similar exercise by changing the grip, angle, etc. The result is a whole lot of burning and a whole lot of growth.