These modes are an integral part of these units, and are important when flying IFR. The enable you to fly multiple holding patterns & circling approaches, without waypoint sequencing continuing. It also provides the ability to intercept and track a radial to/from any waypoint
From the G500 series manual:
The OBS key is used to select manual or automatic sequencing of waypoints. Pressing this key selects OBS mode, which retains the current “active to” waypoint as your naviga-tion reference even after passing the waypoint (i.e., prevents sequencing to the next waypoint). Pressing the OBS key again returns to normal operation, with automatic sequenc-ing of waypoints. Whenever OBS mode is selected, you may set the desired course to/from a waypoint using the OBS Page, or an external OBS selector on your HSI or CDI.
