Sawyer and Kate were captured to be slave labor to help the Others build a runway in the jungle. This is the runway that the Ajira plane would crash land on in Season 6. Why or how Ben would have known that when Kate and Sawyer were captured is another one of those logic flaws in the main story line.
We learned many things about Ben's past. He did not have a happy childhood. His mother died in childbirth, and his father hated him. At a young age, he was taken to a mysterious island. In school, he had only one friend, Annie, who apparently left with a large group (women and children) prior to the Incident. Ben never had a very good social life in school or as a young adult. He did not have the personable skills to charm the ladies. We know that he came across as a stalker with his perceived relationship with Juliet. Ben's jealousy led to Juliet's lover's demise, and Ben's statement that he owned her. But she continued to reject him.
So when a new lady winds up in his camp, Ben takes it upon himself to try to impress Kate. Mr. Friendly takes Kate to the showers and gives her a new sun dress. She is taken to the beach to have a pleasant meal with Ben. He tries to lay on his charm; and infers that he can make her life better if she plays along.
Tom and three Others bring Kate to a breakfast meal with Ben; freshly cooked food, utensils, and coffee, with a pair of handcuffs on the side. Ben tells her to handcuff herself or she gets no coffee. She asks "what did you do with Sawyer and Jack?" But Ben notices that she started the question with Sawyer and not Jack. Kate asks for her clothes, but she is told they burned them. When she asks why he's doing all this, Ben states that he gave her a dress to make her feel "like a lady," fresh food to make her feel at home, allowed her a view of the beach because her friends are seeing the same beach, and utensils to make her feel civilized. He tells Kate that he gave all those things to her so she'll have something to hold on to, because "the next two weeks will be very unpleasant."
This scene is cut off before we learn the actual "details" of the deal Ben wanted to make with Kate. It is one of the holes in the story that could have set the stage for understanding the motivations of both Kate and Ben. We believe that Ben was trying to manipulate Kate in order to get her to convince Jack to operate on Ben. But it seems that this beach meal would have had more long term impact than just a manipulative promise - - - Ben was more effective at getting people to do his bidding out of fear than kindness.
We can infer that there was some mild pass made by Ben to Kate at the beach. That would explain why upon Kate's imprisonment in the bear cage, she had the carnal urge to jump Sawyer. Was this a reaction to a revolting deal ("be my island girlfriend") proposed by Ben?
But we know Kate was just as manipulative with her good looks to control men to do her bidding. In some respects, Kate and Ben are similar loners.
It could have been as simple as promising Kate that she would get off the island after Ben had his surgery. Or that she had to give herself to him in order to save her friends from certain death.
In the O6 story arc, when the helicopter crash survivors made it back to LA, and crazy Jack calls Kate to the airport, I always felt that when Kate said she had to leave to "go back to him," that she reluctantly meant that she had to go home to Ben. (In reality, Kate needed to go back to Aaron.) But if "he" was "Ben," would have been a major plot twist bombshell. It would have put the beach meeting at a whole different level of devilish intrigue. Whose side was Kate on? She got away with murder because she went with Ben. She would lead a rich but unhappy life because that was her deal with the devil. That would be her punishment. Ben would get what he always wanted: a home and family life as part of the deal (abet a loveless marriage).
An alliance between Kate and Ben would have been a devious under-the-radar thread that would have turned viewer heads around like in the Exorcist. Clearly, they would have been the island power couple (which would mirror the theme when Eloise and Widmore were in charge of the island inhabitants).
The Kate and Ben beach scene could have led to much different and dynamic story line than what finally happened in both Kate and Ben's story arcs. Why Kate returned to Jack in the end really does not make sense, since her memories of living with him in LA ended badly. She only wound up with Jack because Sawyer had found Juliet (which itself was odd because it took a time traveling passage for Sawyer to find a woman who made him grow up and take some responsibility.) But nothing was worse than Ben's apparent pining over Rousseau and his affection for Alex. Ben's sideways quest to have a happy life with Rousseau and Alex seems insane, because their island memories of him would be toxic. It would have been quite the twist in the church if Ben did go inside with Hurley, with Ben taking his place in a warm embrace with Ben (leaving Jack and Locke to mirror each other alone in the front pews of the church).
