The Swan computer may be a central hub on the island by means of an intranet as suggested by the Blast Door Map in episode "Lockdown" - the map's straight lines may indicate direct intranet connections.
Each DHARMA station has an identical Apple II terminal linked to a mainframe at DHARMA headquarters. The central mainframe is set up specifically to process the Valenzetti Equation. The scientists at each station input the results from their research into the terminals. The mainframe constantly gathers all of this data, plugs it into the Valenzetti Equation, crunches the Numbers, and broadcasts the results via the radio tower. The transmissions are the only communications between The Island and the Hanso foundation. They serve as an electronic "smoke signal;" once The Numbers have changed, the Hanso Foundation knows that the DHARMA Initiative has succeeded. (See entries on the 1975 DHARMA Orientation Film.)
This is why attempting to contact the outside world could "compromise the integrity of the project." Any distress messages sent from the terminals are picked up by the mainframe, and any "bad" input could throw off its calculations, causing it to broadcast erroneous results.
Or the person on the other end of the computer was at DHARMA central and the test was to see weather the swan occupants had the Initiative in mind and would risk the "integrity of the project" to have contact with the out side world.
The stack of magnetic tapes next to the terminal suggests the computer, (or at least the mainframe), could be used for more than just entering the Numbers.
Ben is the only person who knows the code to enter into the computer in order to display the blast door map. Ben entered an additional code and would not tell Locke that he entered anything.
Ben caused a lockdown because he wanted to get the supply drop for himself, but he didn't think his plan through and locked himself in the computer room.
The lockdown voice countdown was to tell whoever was in the swan that a food drop was occurring.
When Michael thought that he talked to Walt over the computer, he was actually talking to one of The Others.
In the promo for 'Secrets of Lost', a Blu-Ray special feature, Damon Lindelof confirmed that it is Walt, on the other side of the computer.
In the missing section of the Swan orientation video, Dr. Chang mentions that using the computer to contact the outside could possibly lead to another "incident." The incident involved saboteurs from the future attempting to destroy the Swan station. The computer is somehow connected to the past or present, and thus the need for someone to manually enter the code every 108 minutes - it can't be automated. Walt in the future, back in the U.S., somehow figures out how to crack into the computer to get in contact with the Losties (and his Dad) on the island, and ends up communicating with Michael at the end of S2E09.
The Swan computer may be a way of letting the mainland DHARMA to get a confirmation of the project still beeing online. The other stations on the island gather it's own results for their specific fields, sends 'em to the Swan for processing and to be matched toward the Valenzetti Equation to see if the result of the equation changes, and finaly being sent to the mainland via The Flame for further processing. The Swan computer is used to 1): Basically let the mainland DHARMA know that the island DHARMA-crew is alive, and 2): manually enter the Valenzetti-numbers to match incoming data from the other stations once every 108 minutes. This would then trigger an automatic broadcast causing 1):. And so, the protocol is, in combination with the failsafe key, made foolproof and a continuous cycle is created, until either the button isn't pushed or the failsafe key turned.
Communicating with the outside world was advised against as it would ruin the integrity of the experiment, which was as much a Big Brother style isolation experiment than anything else.