Manifestations on the island

The real 'others' can manifest certain things or people to appear for specific reasons.
Jack - Season 1 his dad, who was standing in the water. He later followed his dad to the caves to find fresh water when they desperately needed it.
Ben - Season 3 his mom, the 'others' strategically placed her on the outside of the sonic fence for Ben to cross it and use him as their inside man.
Hurley - Season 2, Dave. This is to deal with the food situation he has. And to help him with his problems with the food.
Kate - Season 2, the horse and Sawyer turning into Wayne. it is a redemption for her to confess why she killed Wayne.