Look at the first photo from Tommy.
Although there is no door, look carefully where the poster is on the wall just behind the raised flap of the counter. There seems to be a "secret" panel in the wall. This helps explain the need for a door and why it is often left open.
That little panel is called a "swinger" - when needed it swings open to allow a camera to get a view from behind the counter. Sometimes an entire wall of a set is made to be used as a "swinger" to allow cameras to get alternate views, where there would not normally be enough space for a camera.
So, having a doorway - and leaving the door open - means there is a convenient place for a camera and as it's a doorway it looks natural - rather than having a slightly suspicious panel in the wall... and it can be used as a doorway as well.
There is at least one episode - I'm not at home, so can't say which - where there is apparently a door to upstairs / outside in the kitchen, when there isn't one just behind the counter.