What are the ANT and ADF modes on an NDB receiver?
ANT mode improves the audio reception when listening to a station's morse code identifier or whatever else is being broadcast.  Normally the directional needle gets parked at 3 o'clock when in ANT.  NDBs can also have other broadcasts besides station ident, like ATIS or AWOS weather data, etc., and in some cases use of ANT may make the difference between making out what is being said, or not.  It may also improve AM radio audio reception, but not sure.  I've used the ADF to listen to music back in the day, but never had to go to the trouble to switch to ANT to hear it.
Here's a link to a KR 87 manual.