The character of Jack Winthrop does not appear in the novel The Last of the Mohicans, and was created for the film. He is portrayed by Edward Blatchford. In the beginning of the film, while Hawkeye, Chingachgook, and Uncas are visiting the homestead farm of the Cameron family, Jack Winthrop is already present. He is recruiting for the British Army, since the French and Indian War is going badly for the British. Jack needs more men to defend Fort William Henry; he hopes to recruit the men of the Cameron household, and any other able-bodied men sympathetic to the British cause.
HAWKEYE: So what is it, Jack? What brings you up here?
JACK: A French & Indian army out of Fort Carillon's heading south to war against the English. I'm here to raise this county's militia to aid the British defense.
HAWKEYE: Folks here goin' to join in that fight?
JACK: We'll see in the morning...
(Mann and Crowe, The Last of the Mohicans, mohicanpress.com)
In both versions, Chingachgook and Hawkeye come across the ambush set by Magua, and rescue the Munro daughters. At that point they learn of the call for reinforcements. In the film, they are aware of the call already, having heard it from Jack, and they later find the Cameron homestead burned and the family killed. This gives the three protagonists emotional depth as they are forced to experience the violence of the War firsthand, as well as condensing the larger history of the War into a shorter scene.
because he want to find a hiding place