They apparently allow more legroom than normal seats given same seat spacing. Of course, that could also translate to more seats with the same legroom. As Thompson says:
Maintaining standard seat width (from 17.8” to 21.0” dependant on aircraft) and overlapping the arms allows us to add an additional column of seats thus increasing seat count by up to 15%, without reducing pitch (e.g. a B777 configured 3-3-3 becomes 3-4-3).
The airline then has a choice to increase seat pitch or keep the capacity increase — or a bit of both. Typically an airline can increase seat pitch (from the typical 32”) to 34” and still achieve a respectable capacity increase.
I’m not sure how these will work for parents flying with small children, though. Car seats must go in the window seat. Combined with the side “wings” on a car seat, this means you’re stuck with a small child that can’t see his parent. Now that’s a recipe for fun! As a parent of two boys who just love to kick (honestly, sometimes I think the hardest part of air travel with a toddler is keeping them from kicking the seat in front of them), I see this as a way to provide two kicking targets instead of one.