The Dura-Ace Di2 SW-9071 attach to a TT/Tri extension providing another location to make shifts. More locations to shift from means you can always be in the most efficient gear and there is no struggling to get back on the aero bars to make a shift. This can save lost time and also increase safety.
The SW-9071 Controls the rear derailleur only and are designed to be used with the ST-9071 Reverse Brake/Shifters. The Di2 SW-9071 Remote TT levers have built-in E-tube wires so no additional wires are required - just plug them into your 5-port cockpit junction.
If you want front shifting capability and/or only want to run one set of shifters on your aero bar set up, use the SW-R671 2-button TT/Tri shifters.
All Di2 components on the bicycle require the same firmware version. Using Di2 components with different firmware versions may cause malfunctions. Use Di2 system checker to update component firmware from http://e-tubeproject.shimano.com/