The water in the keg will be pushing you all around making your body work extra to stabilize in one spot.
We perform these to add some extra load to one side of the body, similar to athletics where there is rarely, if ever, equal load on both sides of the body.
It's also a simple tool for variety.
Perform equal reps on both sides, and if you want to add more work to the lift, turn this into a shoulder - squat combo where you shoulder the keg up on every rep.
That will work you big time.