For exterior doors, these look like they might help if the weather stripping on the door is bad. me, I'd just fix the weather stripping. There should be a seal the door closes against at all edges, though some doors have a bottom "sweep" rather than close against a seal.
For interior doors these might be more useful, but keep in mind that a forced air furnace is designed to draw return air through many parts of the house. Blocking this airflow could reduce heater effectiveness. Ditto for bathroom fans, there needs to be a 3/4" to 1" gap under the door for airflow.