cmp and Urist argue ad hominem, Bjord and Xant argue ad infinitum, Panos and Oberyn argue ad xenophobis and I eat popcorn.
What do you mean by physical world? Electrons and natural laws are human concepts as well.
The outside world would work the same way without humans to understand it.
And I don't know about exists, but I know sometimes you have to ask yourself "ok, so what should I do?". Well, I have, anyway. (Natural) sciences are handy for answering the question "ok, let's assume I did so and so, what will happen next?" but outside of that they're really optional.
It's only science mangled with human concepts that will help you make better life choices. We are not equipped to perceive and reason at a level where scientific knowledge would actually be relevant. We deal entirely in laconic perceptions of huge phenomena and take action based on these perceptions, our memories, our hardwired desires and some fuzzy reasoning thrown in.