We can see the answer to this in Chapter 4 of Call It Courage. In that chapter, Mafatu is excited to find the skeleton of the whale because it will provide him with the raw materials that he needs to make tools.  The book explains this to us saying,

to Mafatu it meant knives and fishhooks galore, splintered bone for darts and spears, a shoulder blade for an ax. It was a veritable treasure trove.

Mafatu had landed on the island with essentially nothing.  He had to get everything that he needed to live. If he could make himself tools and weapons, his life would be much easier.  Therefore, he was very excited when he found the whale’s bones, which he could use to make tools.

Within that same chapter, we see the main use that Mafatu makes of the whale bones in the story.  The first thing that he makes with the bones is a knife.  When Uri, his dog, is washed off his raft and is in danger of being killed by a shark, Mafatu kills the shark with his knife.  Later, he drops the knife and dives to get it.  He then has to use the knife to kill an octopus that grabs him. Thus, the knife ends up figuring in some of the most dangerous things that Mafatu does.

Of course, Mafatu does not know this will happen when he finds the whale’s skeleton. He is just excited because he now has a way to make tools for himself.