They assist publish and push bills that get them seen properly in the eyes of their constituents, and, in the eyes of the nation. Senators stand on the breast of national office and are often jockeying to start further careers into politics with the policies they publish and vote for.
Congressmen log-roll with colleagues. If a republican voted on a farm-reform bill that Thompson wrote, Thompson could vote republican in opposition to his party line to defend his ally. 1 does not