Flightless But Fleet Of Foot
Ostriches are born to run, because...well, they're not built to fly. The Ostrich is a member of the Ratite family, which are birds that can't fly because they do not have a keel shaped sternum, to which the pectoral muscles would attach to power the wings. Many birds of this order have flat chests, and without wing power, have developed into speedy runners.
The muscles of their legs became hyper-developed, and in the Ostrich, the fact that they had only two toes, enhances their speed. The Ostrich can run at up to 45mph in bursts, or maintain a median speed of about 30mph over distances. The feet are part of a unique defense system, with a four inch sharp claw, and enough strength in their long legs, to allow them to kick larger animals like lions, to death.
If they can't fly, why do they still have wings? Mostly for courtship rituals, and displays of attitude. An Ostrich with its wings extended is either showing off for a female, or indicating its territorial nature. They also extend the wings and use them to balance, while running.