Chapter Fifty-Seven explains the way in which Pi determines to not only try to survive his ordeal, but also to tame Richard Parker and to keep him alive at the same time. In his explanation of this he establishes a curious symbiosis between himself and Richard Parker that is only fully explained--in one sense--towards the end of the novel. Note what he says to justify his decision:
It was not a question of him or me, but of him and me. We were, literally and figuratively, in the same boat. We would live--or we would die--together.
Pi therefore devises "Plan Number Seven" in order to keep Richard Parker alive, but of course, it also gives him a target or an aim as well to keep him focused and distracted from his plight. As Pi also confesses, having company on that boat was very important, even if that company was a tiger, as it prevented him becoming the prey of a creature even worse than the tiger: despair.