If the publisher decides that a song is marketable or pitchable, a demo session is scheduled. This helps to keep the costs down.
Some publishers have in-house studios and simply hire in the players and singers.
Other publishers go to a studio. Often in country demos, a pedal steel or fiddle is added for effect. Occasionally, a stripped down demo is done for ballads. This will help save a lot of time and effort in the studio in case you rewrite the lyric and want to re-demo the song. Once the demo is completed, the songplugger at the publishing company hits the streets with the song hoping to get it recorded by an artist.