They greatly improve the aerodynamic profile of the bike. It's what helps reduce a lot of the drag when you start going faster. Bikes in general have a greater drag profile than cars do for their size so every little bit helps. Fairings in later model bikes like ours also work to funnel air efficiently through the radiator and focuses it through the front much better while creating a channel to help it efficiently direct it out. Lastly, fairings are a barrier for the frame and engine. While it may not seem like much, the fairings can absorb quite a bit of energy and redirect it from the structural members of the bike in a minor crash. It's why it seems the fairings almost shatter when they get a good hit on them. It disperses that energy throughout the shell as much as possible before allowing it to go through to the frame.
FreelancerMG's post explains it perfectly. Fairings are not meaningless, or purposeless! In fact, they are very important. To absorb and disperse impact energy is important, and the aerodynamic gains are part of the reason why our mileage is so good.
If you want to prove the point, take your bike on the highway with all fairings off, and then repeat the same course, at the same speeds, with the fairings back on. You will notice a HUGE difference. The bike's performance will be better with the fairings on, for sure.