It's an impartial audience, in a way that your friends and family aren't. Sometimes, you need an impartial party to give a fair assessment.
One of the big problems with posting to reddit is all the assumptions and snap judgements people make.
As a very simple example... I mentioned a couple days ago that I wasn't sure who I was going to vote for in the general election. And somebody responded by telling me, basically, that I needed to make sure to vote locally. Not bad advice. But it's also not useful, since I already vote in every local election. But /u/Alephz didn't know that and made an assumption.
You see this all the time on reddit. People are all biased to one degree or another. We all carry around our own baggage. We make assumptions. We jump to conclusions. Here on reddit we're all responding to nothing more than a couple paragraphs of text and maybe a cute picture. If you're seeing advice from somebody you know in real life that can at least be colored by their history with you. They don't have to make quite so many assumptions.
And you're also getting a world's worth of experience from others, with a much greater depth and breadth than your own small social group may have.
Maybe.
Pretty much any forum is going to suffer from some kind of selection bias... There'll be some type of person that frequents it... Slashdot, for example, probably has more neckbearded IT workers reading it than the Huffington Post does.
You ever notice how many Finals are Coming memes show up on reddit every year? That's because there's a lot of college kids not only posting to reddit, but upvoting those comments.
Just because it's an on-line forum does not automatically mean you've got some kind of broad, across-the-board appeal. It's still advice from your social group. It's just your on-line social group, instead of your real-world social group.
My friends were in the same position I was
And that's not necessarily a bad thing. Out of context advice is not necessarily helpful.
As another anecdote, I'll mention my father. He's a retired optometrist who has done quite well for himself. Finances simply aren't an issue. And he's basically useless when it comes to advice because he and I simply do not live in the same world. I remember randomly lamenting about having to wake up early and shovel the driveway last winter... And his advice was that I should hire somebody to clear the driveway for me. Which would be very nice, I'm sure, but I just don't have the disposable income for that - which makes his advice useless.