HN is how I stay current in my field. Back when I was doing some sysadmin stuff for work, I shifted to writing scripts in Python instead of Perl because people on HN kept talking about it and I wanted to see if I needed to learn it. Knowing Python has broadened work opportunities, even though I mostly do C++. HN gives me a way to monitor up-and-coming trends, and decide if I need to join.
I got a fantastic client from the one who's freelancing post I did. It was one of those where the appreciation was mutual, so much fun to work with them!
Somebody posted the history blog (acoup.blog) last year, and I've learned so much from reading it. I've been tossing around writing a fantasy novel at some point, and due to his writings and explanations, I realized that the Tolkien-quality fantasy wasn't so much a product of twenty years of pre-writing like I thought (although that is part of it), but actually understanding how ancient people thought. This gave me guidance for research: I, too, need to learn how people in the middle ages thought and lived if I am going to write a believable middle-ages fantasy. Too many authors put modern characters in the middle ages, and it just doesn't feel right.
Patio11 made consulting/contracting seem like something I could do. I'm also indebted to his article on raising your rates (which I can't seem to find the original of, where he talks about hyperventilating over pressing send on his first email to raise his rates).
Reading all the blogs linked here made me realize I needed to start a software blog, kind of as a marketing tool (in the way that HN is a marketing tool of YC, which it isn't really, but it does have that side effect). No tangible benefits yet except that thinking about it has caused me to understand things better in a way I didn't before.
But don't ignore the intangible benefits, which is why I keep coming back. I get exposed to all kinds of new ideas. I wouldn't care about mechanics of startups, but Steve Blank's hilarious stories kept me reading even when he transitioned to less exciting didactic stuff. No plans on starting a startup, but if I do start a business, I am so much better informed about it because of years on HN.
And there's always a couple comments a year that change how you think about something. I don't know where else I would find that.
(By the way, thanks @dang et al for moderating, it really makes a difference)