This timeline option was added in version 23.11. This option automatically adds an empty audio or video track to have ready so you do not need to add them manually. An empty track also helps to provide some space in order to select by dragging a box or rectangle or to provide more area for scrubbing.
It only adds tracks when you add something to the timeline or move a clip between tracks. Also, it does not automatically add a track when you open a project with no empty tracks.
In addition, this can be helpful to new or infrequent users who do not know you need to add an audio track or how to do that to add music. Or that you need to add a video track to overlay an image or video on top of another.