This operation does nothing. It acts simply as a placeholder to make wiring networks easier, or to allow you to reference a single output SOP without having to update all the references to it if the real output SOP changes.
This SOP passes through its input untouched, so it does no cooking. Adding a null does not increase memory usage (except when "Cache Input" is on).
Disabling this flag will cause the Null SOP to cook its input, but not copy the result. This is useful if you want to enforce the cooking of a node at a certain time, but are not interested in the result. You can use a Merge SOP to collect a lot of dependencies into a single input this way.
If this flag is turned on, then the null will cache its input. This makes it possible to unload the Null SOP’s input. This flag is off by default.