Better power transfer - clip in shoes allow to you generate power the drivetrain whilst pulling up as well as pushing down and cycling shoes are constructed with a stiff sole to once again increase power transfer whilst pedalling
Comfort - cycling shoes are specifically designed to provide support where your feet need it during the pedalling stroke which helps to minimise muscle cramp and to keep your feet cool and dry through use of breathable materials and drainage systems
No slipping - your foot is held in the ideal position over the pedal axle