The point of view of this chapter is unclear; it could be Ishmael narrating, but the complete lack of first-person pronouns and the insight into Ahab’s mind both suggest that this is our third-person omniscient narrator.
Even though Ahab is completely obsessed with Moby Dick, he’s still interested in hunting other sperm whales, partly because he’s always been a whaleman and partly because continuing with the original goal of the voyage (to make money) will keep the men (especially Starbuck) in order.
The seamen need something to do other than stand around obsessing about Moby Dick all the time.
They also need to feel like they’re getting rich—or at least making a living—off of plundering the sea, the way crusaders pillaged as they traveled toward Jerusalem. (That’s Melville’s analogy, by the way, not ours.)
Ahab also wants to make sure that his crew and officers don’t start thinking too hard about how insane he is, because if they mutinied he wouldn’t be able to carry out his plan for revenge.
The upshot of all this is that Ahab gives standing orders that they’ll hunt any whale they see. (Otherwise, how could you have more than half the book left to read?)