People with a "second brain": Why? How?

I've been looking around at different note-taking apps (Notion, Obsidian, Anytype, Joplin, Logseq, etc.) after seeing a few videos about the idea of having a "second brain", and only how has the the thought popped into my mind, "What's the point?". A “second brain” seems like it would require spending too much processing power on something that only exists to exist. What are the chances there you're going to remember any given thing after writing it down? You haven’t filled up your first brain yet. This all sounds very pessimistic - my intention is not to insult anyone's choice to use these tools, but I'm curious of what benefits people have gotten from their "second brains". Maybe I'm just the wrong kind of person for it, or maybe it's just that I'm not used to writing things down.

Edit: I'm coming to an interesting conclusion that many people use their version of a second brain for things they need to do. This isn't really what I was referring to, I was more looking at it as a form of journaling or personal research, which might be less common?