They help a whole bunch for the IAB (Invisable Aero Bar) position where the forearms are rested on the tops. The flatter tops don't hurt as much and are often taped/padded to support this particularly in Merckx type races.

Just riding - I expect very little bit. I'll make it up having seen my kid do testing Merckx with and without them.
~5-10 sec @ 27 mph over 30 min with hands on drops. I'd say position (IAB) is good for 1-2 min.
That again is a made up educated guess number, but they are not as important as wheels, or helmet.