In Life of Pi, Richard Parker helps keep Pi alive on the lifeboat by killing another dangerous predator, the hyena. However, the tiger himself is an even more dangerous predator and, despite his seasickness, poses a constant threat to Pi on the small lifeboat.
Paradoxically, this threat helps to keep Pi alive, and it does so in two principal ways. First, Pi is in a dangerous environment even discounting the threat posed by Richard Parker. He is in a small boat on the open sea, vulnerable to many forms of danger which may not be immediately obvious, since they would come from outside the boat. Having a constant danger on the boat helps to keep Pi mentally alert.
The other way in which Richard Parker keeps Pi alive is related to this. Even experienced sailors in Pi's situation are notoriously likely to succumb to madness or despair. Left to brood on his situation, Pi might very well become so despondent that he decides life is not worth living, or start drinking seawater, or doing any number of other things harmful to his own well-being. Again, Richard Parker keeps Pi's mind focused and in the present, keeping insanity at bay, and providing Pi with a curious kind of companionship.