As was stated in the Season 3 episode "The Glass Ballerina" it was a decoy village that Sayid found. The door was just a decoy of a hatch, there as NOTHING special about it. The Others just wanted to give Sayid and Co. more reason to come investigate the village and throw them off the scent of where the Barracks are located.
The fact that the entire village outside was abandoned, not just empty, but cleared of all of its contents, means both the door and the village are just meant to keep up the charade of the Others.
Likely given the subsequent revelation that it was a "decoy village". The Others thought that their village would be more credible if it had its own DHARMA station.
The fake door is a copy of the Hatch of a real station or The Door may still be a hatch/station on the Island, just not at the location of the camp. The Hatch doors may have been scavenged from another station entrance.
The Others set up the fake hatch door as extra bait for the Losties, having noticed their curiosity about the Swan and Pearl. Hearing about another hatch would encourage the Losties to make a visit.
The area right behind The Door might, at one time, have housed another pocket of exotic matter, similar to the cavity in the Orchid. Either Sayid didn't notice this when he inspected the door, or some event in the past (like a previous moving of the Island) has rendered this exotic matter inert.
The door was one of the exits that could be used to leave and reenter the Island.
It is also the "aborted #7" referenced on the Blast Door Map. Construction had begun, but for unknown reasons was never completed.
The rock behind The Door is a façade and there is a way to get past it.
This is supported by the fact that Sayid never actually touched the rock wall. He merely looked at it and shut the doors again. If he were to press on it he would find it was a less solid material than rock and he could enter.
There is something similar to a "Garage door opener", a remote control that opens/removes the façade so you can enter and use the station.
The Door is a meteorological station, as meteorology was a major study of the Initiative.
The Door is a psychological research station. The rock wall is an experiment, and there is some way to get past it. For example, it serves a similar purpose to the blast doors inside The Swan. In fact, they are blast doors that are made of rock, or built to look like it.
The fact that there were guards outside the door was a part of the experimentation going on inside.
As its name suggests, The Door is actually just meant as a door. Its purpose is as an access route to the Barracks, avoiding the Security Perimeter. This is supported by the fact that the Others must have some way to get around the sonic barrier as it is unlikely that they carried Ben over the structure in his stretcher in the same fashion that Locke, Sayid and Kate did in "Par Avion"
No, it is an exit from one of the subterranean conduits mentioned on the blast door map.
The door was originally used to enter the (now flooded or otherwise blocked) tunnel (think pneumatic tube) that connects the main island to the Hydra station. The tunnel is also the reason why Hydra island stayed with the main island while it was being moved. The tunnel/tube tethered Hydra to the main island, similar as the cable did with the Looking Glass station. Remember that unknown Dharma symbol with something that looks like a tunnel?
Man In Black
The Man In Black and his people created the door as a passage to reach The Source just as he did with the well and the wheel. However it was blocked off by Jacob just as Mother did with his well in order to sabotage his plan.