To be real
There are a lot of things we believe, but only some of them we believe completely and unflinchingly, like that when we drop something it falls to the ground. Others we don't really believe, or we only believe in a kind of consequence-free way. Beliefs like "everything is connected" are unfalsifiable and thus consequence-free, but you can also find other less obvious beliefs like "I'm going to get in shape" or "I'll backpack around Europe someday". They look like real beliefs, but if you don't actually act on them and they don't have any consequences then they're not real.
The inimitable Kurt Vonnegut used the idea that an entire religion could be built out of "harmless untruths". The kinds of unreal beliefs, like ghosts or the Loch Ness Monster, that don't really have any consequences. Now that you believe that ghosts are real, what are you going to do differently? Are you going to buy special ghostbusting equipment? Unlikely. Probably you'll go through your daily life exactly as before but occasionally say "I believe in ghosts" and that's that.
In fact, you can tell if a belief isn't real because attempts to make it consequential are amazingly uncomfortable. If you act like someone's belief in gravity is real, for example by challenging them to drop some stuff, or betting them money that an object will fall upwards, they'll happily do it – who doesn't like free money? But ask someone who wants to get in shape to tell you their specific plan, or bet someone who wants to go to Europe a lot of money that they won't go by a certain time, you'll quickly get a picture of whether their belief is real.
Some unreal beliefs are better off discarded, but there can also be a lot of benefit in reforming them into real beliefs. Maybe you really do want to backpack around Europe, and the fact that you haven't made any plans, researched places or drawn up a budget would be quickly rectified if you felt like it was really happening. In fact, that's kind of the definition of real: something that actually causes you to act or decide differently. If your goals and aspirations aren't real, if you don't feel like what you're doing is real, there's no reason for you to try to succeed.
Paul Graham did a great bit on why startups die, where he points out that while on the surface startups die from running out of money or a founder leaving, the root cause is usually that they've given up. They don't go out in a blaze of glory, they just sort of shrivel up and disappear. My reading is that, for the founders, the startup stops seeming real. They might still say the words, but they stop acting as if it's going to succeed and, like an imaginary best friend, it just eventually vanishes.
This, I believe, is the secret sauce behind the famous Steve Jobs Reality Distortion Field. It seems like being out of touch with reality would be seriously maladaptive, but evidently it worked well for him in business. I think the clue is in the name: when Steve believed in something, it was reality. He acted like it was true and he convinced other people to do the same. And that shared illusion was necessary for the ideas to succeed.
Though notice I say necessary and not sufficient. Bluster isn't belief, and you can't make something real just by believing in it. But it is necessary to believe in what you're doing. More than that, it's necessary that what you're doing is real: not just the kind of thing you talk about, but something you act on and rely on as unwaveringly as gravity.