When an individual turns on a bitcoin node, they express their will for what the rules of bitcoin should be, instantiated with robust software that allows for no exceptions. They need not turn the node on for any reasons other than those of pure self-interest. The node is not meant to add security to the network in an altruistic sense, where an additional node adds to a particular security metric. Instead, they add to the network an expression of what defines a unit of bitcoin its user will receive and spend.