Because calling it a computer means you can apply computer science concepts to it. That's the interesting part. You can talk about the classes of algorithms the brain uses to solve difficult problems, the ways it tries to conserve energy, the architecture trade offs, how to model it with math, etc. That stuff is non-obvious if you treat it like a magic black box. It's not a meaningless insight

Can the brain solve NP problems in polynomial time? The brain itself is a mystery, but we know some things about computation, so it's pretty safe to say no, it cannot.

Maybe on HN it's consensus that all living things are computers in this way, but plenty of people think the brain is literally supernatural. By saying "the brain nothing in common with a computer. We don't know what it's doing", those people will be nodding their heads. But we do know some things about computation, so we can apply those insights to the brain