The locater is a requirement in certain situations on certain ILS approaches. If an ILS approach has an associated locator you must have an ADF unless a different procedures is available to intercept the ILS i.e. a DME Arc or Radar Vectors.
An example may be Canberra 35 ILS. If you go in there at night and the DME is u/s you Must have an ADF to intercept the ILS beacuse the approach frequency and tower is Closed.
Also on top of this the ILS must be intercepts prior to the Outer Marker.