Early in life you get the impression that parents know everything, then it's teachers, then it's bosses. When in fact they don't.
We're not robots with perfect abilities, understanding that everyone to a degree doesn't know what they're doing gives you a different outlook on the world.
Note, that doesn't mean everybody is incompetent.
How can you possibly make this assertion? I have a direct counterexample; I know what I'm doing. There's a lot of stuff I don't know, but I would not describe my state of being as "not knowing what I'm doing".
>everyone to a degree doesn't know what they're doing
This is very different from "Nobody knows what they're doing". Just because no one has a perfectly clear idea of everything doesn't mean they don't know what they're doing.
EDIT: Actually "all the time" doesn't quite cover it, I'd say it's "most of the time".
... Er, no. There's only so much complex, useful stuff one can produce, before you have to conclude, "okay, that person understands something."