The EMF may be a security system for the Man in Black; just like sonar fences stop him to move in one direction, whole EM around the island prevents his escape. However after Desmond activates Fail-safe, he is still trapped (as far as we know) unless Jacob dies; this may be like the double-security (one being ash other being Dogen) in the Temple.
One theory is that the island gives off a strong magnetic field which creates a strong north-south field in concentric rings around the island, just like a normal magnet. Traveling across the rings (rather than due "north" or "south" from the source) would slowly change the heading. Trying to stay on the same heading would result in sailing around in large circles around the island. The heading of 325 could be the "magnetic north" of the islands localized field coupled with the earth's field (making the "true north" of the island pointing about 290 degrees).
The high EMF(electromagnetic fields) is part of the island itself. The Swan station was built to manipulate the EMF's so that the properties of the island can be controlled. People in the real world have been known to experience symptoms due to high EMF's usually called electromagnetic hypersensitivity, symptoms include: skin irritations, paranoia, fatigue, hallucinations, and many others. Hypersensitivity could also be the sickness that is commonly referred to in the show. Also there are theories that ghosts and the paranormal can be detected through EMF's and one can use EMF's to help manifest itself to the living. This would explain why the island seems to be able to absorb the dead and use them to do its bidding, i.e. Christian, Yemi, and Kate's horse. High EMF could also explain the monster since in the real world some have said that they see a black mass near an electrical output that emits high EMF's and they always feel uneasy near that place where the high EMF's are being emitted from. All the high EMF from the island could be the very reason why it is so special.
It has been said that Desmond's use of the fail-safe prevented a potential global catastrophe, per the producers (see Fail-safe/Activation). We have been told that the result of the Man in Black's escape from the Island could be the destruction of everything, a global catastrophe. The MiB's escape plan involves a massive buildup of electromagnetic energy which, when it reaches a critical level, will culminate in his release from the Island. This is what he means by "channeling the light and water" in Across the Sea - his goal is to focus the water and light in order to intensify the electromagnetic energy until it reaches critical. This is also part of why Jacob keeps bringing people to the Island; they must prevent the MiB's attempts to achieve such a task. Dharma started discharging the energy gradually because they saw that a buildup of it could cause a cataclysmic destruction. The MiB somehow persuaded Locke to destroy the computer, resulting in this nearly happening. If not for Desmond turning the fail-safe, it would have been game over.