nnnil is a backend that can be used as a placeholder if you
have to specify a backend somewhere, but don’t really want to. The
classical example is if you don’t want to have a primary select
methods, but want to only use secondary ones:
(setq gnus-select-method '(nnnil ""))
(setq gnus-secondary-select-methods
'((nnimap "foo")
(nnml "")))