Maybe at the moment friends are unnecessary for him, because his family provides his needs. So a practical reason would be that when one becomes an adult one doesn't always have one's family around, and it's important to have other people around who can help you out. But of course it's important to point out that friendship goes both ways, and he'd be expected to help his friends out too, or they won't want to be friends with him.
I think the emotional benefits to friendship are not something that can be explained to someone who hasn't experienced it. If he experiences friends at some point, he would understand more clearly.
For me personally, I like to have people to discuss issues with, to bounce ideas off, to hear other perspectives, to share experiences. I am a very curious person - I like to learn about the world, about other people, about how people's minds work. It's also a safe feeling to have people who care about you. I find friends can motivate me when I have difficulty with self-motivation.
Actually, thinking about it, there is a quotation that once made me see how important friendship is - it gives a good analogy: 'Though one may be overpowered, two can defend themselves. A cord of three strands is not quickly broken.' It's from the Bible, and actually, it was reading that passage that first made me really make an effort to make friends.
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'If the shoe doesn't fit, must we change the foot?' Gloria Steinem