According to Reuters, this hypothetical mode would allow customers traveling through an emissions-free area to avoid running afoul of environmentally-focused laws and regulations.
This is just an idea on BMW's part at the moment, but it would probably be fairly simple for the company to roll the feature out to its more modern hybrid vehicles; perhaps in the form of an over-the-air software update. After all, most of BMW's newer vehicles are already filled to the brim with connectivity-oriented functions.
Even if zero-emissions zones never become a global reality, giving drivers the option to choose a greener method of driving from time to time -- even if it comes at the cost of range -- could only be a good thing.
Of course, it's worth noting that even if BMW gives its customers an option like this, there's no guarantee that cities will let drivers off the hook. The end-goal of emissions-free zones is undoubtedly to push people to purchase and use fully-electric cars, and an electric-only mode for hybrids may not be good enough for the more environmentally-conscious officials out there.