My theory about Walt and his special abilities by Sandra

So, we know that Desmond possesses special abilities to withstand high volumes of electromagnetic energy, time travel while retaining consciousness of the present, etc. Did he get this ability from the island? No, right? There is just something special about him that he must have been born with.

Same is the case with Walt! Walt was born special. He showed his special powers before ever setting food on the island. This is just something we have to accept as part of the Lost mythology; that people can just be "born" special, and that's that.

Now, we also know that every single one of the Oceanic passengers/survivors was brought to the island for a reason. Not just the candidates. When the plane first crashed on the island is it more than likely that there were a lot more than six candidates. For example, we know that Kate was a candidate until she became a mother ("What They Died For").

So, my guess/theory is that Jacob knew of Walt's special powers and brought him (and possibly Michael) to the island as a candidate, hoping that Walt's powers could be used against the Man In Black, in protecting the island. Walt was a candidate at that point, and Jacob saw potential in him being "special".

HOWEVER, as we learned in "Ab Aeterno", one of Jacob's key principles is to never take away a person's free will. Therefore, Michael was free to leave the island - with Walt. Walt was simply crossed off the list of candidates.