It makes sense when you consider what turning on means for the state of the device. Almost all electronics have a default startup sequence which places them in a very specific and organized state, which is thoroughly tested and stable.
During operation, as devices change states, they can build up things in memory (and for devices which run applications, those applications enter states of their own, which can conflict between applications, etc) and in the absence of tools to return devices to a clean state, restarting will correctly guarantee a stable state of operation.
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