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What are the benefits of clipless pedal systems on road bikes? - shimano road shoes

I have the Shimano road shoes, but I have a dilemma:
I'm scared clipless pedals and the fact that my feet are attached to the bicycle. I know I will be with the practice in a position to use freely, but I'm afraid to start.
Can I just walk in shoes without spikes? Negatively on the efficiency of my pedal? How many additional muscle work in the case of the decision for a walk without shoes?
Thank you.