Good stairs have good landings. It only makes sense from a home safety standpoint. A landing, a level place at the top of a staircase, assists in traffic flow and prevents falls or run-ins when opening and closing doors. In most places, landings are required anytime you have three or more stairs. More than three stairs apparently equals a height that's dangerous when it comes to falls.
If a door opens out over the landing, general consensus says the landing should be large enough, usually 3-feet by 3-feet, so that anyone standing on the landing isn't knocked off when the door swings out. That only makes sense.
Landings also play a roll in keep rain and other moisture out of the house. To prevent water from coming in, exterior landings should be lower than the threshold of the door to the inside. Landings that are flush with the door provide an area of weakness where water can seep in onto interior floor surfaces. The height of the step up into the house depends on the region and weather conditions.
Of course, stairs and elevated thresholds are not ideal for people looking for a home with Universal Design principles. Universal Design principals allow the home to be easily adapted to accommodate someone in a wheelchair or needing additional assistance. In these cases, thresholds shouldn't be more than ½ inch higher than the floor. Accessible doors should can be covered to keep wind and weather away from the opening. Covers also help prevent ice from accumulating near the landing or stoop and becoming a safety hazard.