Because it tells you that's what "society" thinks of you. And that will affect your day-to-day life.

It affects folks in the same way learning that a lot of folks on Reddit/HN like to Haskell. "Oh hey I didn't know Haskell was so popular"

But "you" are not "you" when you post on an anonymous imageboard. Most boards don't even have poster IDs, so you can't even distinguish individuals with multiple posts in the same thread. You're just seeing a bunch of posts from random people who could even be replying to themselves, and you have no way of knowing, unless you obsessively check the "unique IPs in this thread" number as new posts flow in.

I post on HN with my real name, but you use a pseudonym. Nevertheless, your pseudonym has a posting history and karma number attached to it, so anything you post will "stick with you" in a persistent manner. Discussions somewhat resemble real-life discussion because one can track who's saying what, what everyone's said in the past, and so forth. If I flame you, my post probably gets deleted and I might even get banned, because that's how this community has decided to moderate itself.

Anonymous imageboards like 4chan are wholly different beasts altogether. If I post something foolish here on HN, perhaps talking out of my ass, saying something incorrect, I'd get downvotes and comments telling me why I'm wrong. You might even remember my handle as "that one guy who said the dumb incorrect thing one time" next time you see it. on 4chan, when you make a dumb post and everyone dogpiles on you, calling you out for your idiocy and calling you bad names, you can just close the tab and move on in your life. People aren't attacking YOU, they're attacking Post #418317258, and its author has closed the tab and permanently disassociated himself from it.