You forget the ultimate reasoe: because we can.

I'm saying that because it looks like you're going after them for doing so as though it's a bad thing. I suspect you're just confused about the choice because it doesn't make sense to you, and you're trying to figure it out, but aren't aware that the way you're going about it comes off less than friendly

That being said, it's the same as having multiple reddit accounts, or twitter, or Facebook, or whatever.

Redundancy. Backups need backups, which could use backups.

purpose specific options (like only accessing one type of content on a given account so that it's easier to focus on a topic as needed)

enjoying the freedom of other users not knowing that it's the same person, so that you can avoid being followed around (for good or bad reasons on either side) or just escape a more "popular" account's connections connections.

For the fediverse in specific, there's also the fact that defederation exists (which OP already mentioned), so you night not be able to access all instances from one account. As an example, if I want to access beehaw, I have to have a second acccount.

And, with porn in specific on lemmy, a lot of instances expressly forbid it because of local laws, so having an account on one of the two (or both) porn focused instances is a wise choice to prevent any issues in that regard. Piracy is a similar situation, though less problematic overall.

And, again, why not? There's nothing wrong with it, there's no significant drawbacks, and it doesn't cause trouble for other people.

Now, you may or may not have any of those use cases, and that's okay. But others may/do.

Hell, I keep my author account fairly heavily isolated from almost anything else I do. That's another use case; isolating public/professional identities from casual account activities. Nobody that's interested in my books needs, or should be able, to have access to the rest of my online life. It's none of their business. Ever see any of the reports of people losing jobs over their Facebook activities under their real name? It's also very common for employers to do a quick name search nowadays in case the prospective hire is sketchy.

There's way more reasons to keep multiple accounts than there are not to. I'm just surprised that there's people that don't.