At first glance this looks like the mount that fixes the battery to the frame. Upon further inspection you will see it is actually and adapter FOR the mount that attaches the battery to the frame. Here's the explanation: The Di2 rear wire kit comes with an attached battery mount. This mount is a long and narrow alloy track that fits underneath the bottle cage. Is long enough that it actually extends past the bottle cage so the battery can then mount onto it. Now, is some instances this mount will not work due to the frame being too small or due to certain restraints due to the frame tube size, shape or diameter.
In that case the adapter is now necessary to make this work in this situation. The Shimano Di2 cage adapter allows the water bottle cage on the seat tube to be moved upward avoiding clearance and compatibility problems. The adapter provides a quick and easy fix.