I think the Bicycle Transportation Alliance coined the term 'Hand Signals Rock' and I would like to extend it to turn signals for autombolies as well.
Turns signals tell us what our fellow road users plan to do before they do it. Turn signals help drivers not hit other road users they don't see because they give the other user a chance to prepare.
(Hand) turn signals are an important part of bicycling safely. They help prevent you from crashing into other cyclists and from getting hit by oncoming cars.
When we get on the road or trail, we rely on each other for a safe trip. Turn signals and hand signals are an expression of citizenship. If you love freedom, shows us your signal.
Bike signal illustration courtesy of http://letsgorideabike.com