>Why does the universe exist at all?
Wrong forum for it. But sure, I'll bite.
Philosophically speaking "nothing" implies the absence of "something". You can't have one without the other. Meanwhile, our very existence proves that a mathematically almost impossibly unlikely set of variables have come together to create the universe. Even a tiny little change to any of countless fundamental properties and we could not exist. This implies that whatever started the big bang, must be a recurring phenomena that is fundamental to the multiverse, existence itself. But nature is always simple. From fractal geometries to the standard model, all the mind boggling complexity of the universe itself is derived from a tiny little few simple things. Just a few forces, variables, that once given enough time and energy and repetition, creates everything we know.
Ironically, we already have a concept that explains this. Binaries. Everything you do on a computer, every software, audio, image, every complex functionality the entire internet, is all just 1's and 0's. Which takes us back to the start.
It's more than likely that the base nature of existence itself is just 1 = "something" and 0 = "nothing". Just static, if you will. And occasionally, when the static arranges itself just right, it manages to create a kind of code, that can birth time, matter, energy, and the simple physical laws that govern all of it. And that's what we are. A fluke, that will infinitely repeat itself in an infinite number of different ways, forever.
>What the hell are we for?
What is an insect for? It just is. Governed by its natural instincts it tries to eat, breed, survive. We are no different. There is no purpose. There is no reason. We exist until we don't. What you do with it is your own choice.
Some people believe in invisible God men to give them that purpose. You can make up your own if you like. But scientifically speaking it's pretty obvious there is no such thing.