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Though the idea that the nervous system generates outputs without inputs seemed strange at first, it actually made a lot of sense to me once I thought about it.  Many of the behaviors that puzzle us, including many of the things that we define as mental illness, like depression, schizophrenia, OCD, mania, and panic attacks, can't be explained by a traditional stimulus/response explanation of the nervous system. They happen without any external cause, and the pharmacological treatments we have for them are based on changes within in the nervous system.

The fact that the nervous system can generate outputs without inputs from the surrounding environment is interesting from an evolutionary perspective. The nervous system is capable of changing how it reacts, because any one output is not linked to a specific action. As result it can adapt to a new environment more quickly and easily than a system that is based on linked inputs and outputs. It also introduces an element of the unexpected, because outputs that occur without inputs cannot be reliably predicted. Variation drives evolution, so in evolutionary terms the nervous system is highly valuable. It also begs the question of how such a system evolved. What was the starting point for outputs without inputs? Did it start as a stimulus/response system? Or as something else entirely?